I installed a snapshot of CM 11 (KitKat) onto my device with little issues. I'll update this guide with the links to that firmware, and also update the existing links because of changes CyanogenMod made to how they aggregate their S4 devices now. They are treating them all as derivatives of 'jftle', so basically one firmware fits all devices, including the i9507! (even if not official yet).
Please read the bottom of this post if you wish to install KitKat on your device.
CUSTOM FIRMWARE ON SAMSUNG GALAXY S4 i9507 - Aus/HK
Tonight I have managed to root the device's stock firmware (I have the Australian Optus/Virgin device); install ClockworkMod Recovery on the stock firmware, and then use it to install CyanogenMod 10.2.0 (jfltexx) on the device.
If you're looking to root this device, I found that by treating it essentially as an international i9505 I was able to fulfil all three of the things I mentioned above.
UPDATE: THIS WILL VOID YOUR WARRANTY AND RESULT IN 'KNOX WARRANTY VOID 0x1' BEING DISPLAYED WHEN YOU ENTER DOWNLOAD MODE ON THE DEVICE. PLEASE BE SURE YOU ARE OKAY WITH THIS HAPPENING BEFORE TRYING THIS FOR YOURSELF. AS IT STANDS, KNOX CANNOT BE RESET, IT IS AT A HARDWARE LEVEL.
Using Chainfire's guide to CF-Auto-Root in the Galaxy S 4 i9505 Original Android Development section of this forum; I was able to root my device despite it being an i9507. Being a new user, I wasn't sure exactly what things to test out whether the root worked/was permanent, but I did install AdBlock Plus and it asked me for Superuser privileges, then proceeded to work. I also restarted the device and tested whether the root was still present; it was.
I decided to install CyanogenMod as I have heard from friends that have it; and read online about it being one of the more stable custom ROMs available. It has been a while since I last owned an Android device! So, continuing with my theme of treating this device as being an international i9505, I obtained CM 10.2.0, a stable release of the jfltexx CM, based on from their website (as of December 2013, this was the most recent). Their custom installer app on Windows did not support my phone, so I had to use their guide for manual installation.
Their guide contained steps to installing ClockworkMod Recovery as well as the ROM installation. Following the guide, I installed it without a hitch. When rebooting the phone from ClockworkMod, it prompted me about the fact that the phone may reflash its own recovery tool, but this did not happen. Once again, a success.
The last task was to install CM. This was quite an endeavour as I decided to use someone else's Windows 8 laptop (being the only Windows device I had access to at the time) and I forgot how much of a struggle it can be! Again, I used CM's guide to installing 10.2.0 (which by the way is Android 4.3.1, whereas the stock firmware is bloated and heavy at 4.2.2). This went off without a hitch once I reminded myself how the ADT works. I also did as CM suggested and installed the Google Apps pack alongside the ROM.
So, currently this is where I'm at. The device boots and reboots fine. Some users in certain threads had issues with loading 4.4 (KitKat) ROMs or custom ROMs that they didn't allude to the names of, so I don't know if they were having issues with CM or not. There are other threads, but that was the one that kickstarted my attempts to get this all to work. Their issues with the custom ROMs seemed to be related to the 4.4 ROM not recognising the modem of the i9507; I'm not sure why this is but the i9507 was made specifically for Australian/Hong Kong networks so perhaps 4.4 has not been updated yet or something of the like. Anyway, on 4.3.1, or CM 10.2.0, my SMS, calling, and internet are working fine (it displays Virgin Mobile as my carrier). My 4G reception is somewhat iffy in this town (or in particular my house), and the reception seems to be switching between 3G and H so whether that is indicative of 4G not working, I'm not sure. LTE is definitely checked in the settings, however, and my area does have 4G, but my parents' iPhone 5Ss sometimes do not have 4G in this house either. I'll report back tomorrow after having tested the phone for a day to see if the 4G does activate.
As it stands, however, the phone seems to be functioning well. Oddly the Settings report that the phone is an i9505, I assume that's normal as I'm using a ROM for that particular phone!
So, I hope that anyone who has been looking for a way to remove the carrier/Samsung bloat from their i9507 device will have a good time using the guides I've used. I followed them to the letter and I had no issues.
DISCLAIMER: I had no troubles, but that's not to say you won't either. Please be careful, as you may potentially risk affecting your phone negatively. I'm simply telling you what I did, not giving you a 100% correct and flawless guide that will work no matter what.
Also if you're looking for a 4.4 ROM then you might be out of luck, least till some dev or coder takes notice of this device's existence and helps out to develop a specific way of rooting this phone/installing custom ROMs for this phone. Overall though, it seems to all work fine so perhaps the 4.3.1 CM isn't such a bad thing.
If you try this and want the original firmware back on your phone, use this link. I believe that OPS is the Australian firmware, the other one is for Hong Kong. So, for the second link, HK is Hong Kong, and the one without is for Aus.
I've successfully managed to install a Nightly build of CM 11, based on KitKat. Read my latest post in this forum for more info. I essentially found that upgrading from within the phone itself caused a number of issues so I did a fresh flash. Please note that such a flash of a new OS wipes all personal data but not things like images, videos, etc. I lost all my contacts, SMS, browsing history, apps etc. But my Gallery, Downloads folder, etc remained intact.
I followed my guide to flashing the phone again, but I used a Nightly build of CM 11, based on KitKat. As I write this, there are actually stable versions of CM that you can use in place of the CM 10.2.0 I used in the above guide. If you follow those steps but use this firmware you'll find no issue. Feel free to use CM's guide, but don't, under any circumstances, forget to flash Google Apps to your phone!!!
If you need a hand or want a question answered, feel free to post here and I'll do my best to help you out. Thanks!