[Q] Advice on tweaking my Tmobile T999 S3 to use on ATT now?

Hello, Just changed from Tmobile to ATT. I already unlocked it, and am running it on 4G. Sad part is, download is only like 1-3 Mbps it seems. While my Iphone 5 can get close to 7 Mbps (Now this is just 4G band, NOT LTE)

So I am wondering if there are some tweaks I need to adjust in the build.prop or anything else. Atm I just use the stock rom thats rooted. (Since it seems nice enough for JB and honestly, I havent figured out which custom rom beats the stock rom 100% without losing any features.

Also noticed I cant install the ATT Visual voicemail as it claims device isnt compatible. (Which that I am sure I can use a build.prop edit for the model. Question is, which model and items do I need to adjust and to what value?)

Anyways, if anyone has some good advice about using the tmobile S3 on ATT and getting it tweaked to be the best it can be, or a rom that is perfect for switching between ATT and Tmobile (should I use my prepay tmo sim in it again) and provide high data speed. Would be great.

Oh also.. just so I know. Can I use the ROMs posted in this ATT section of XDA for the t999 tmo s3 or do I need to still only use roms from the tmo s3 section of xda? Thanks. =)

Posted there too, not much luck on responses yet. So figured maybe this side knows more. I dont even know if one can use the ATT Xda rom side for the tmo s3 for example. Was there any hardware differences (aside from the radio since I know tmos doesnt have LTE) Also what is the model for ATT s3 for the build.prop?

Anyone least know if the roms in ATT section would even work on the Tmobile model? Kinda thinking I should switch out and have all the setups and apps for ATT.