Luckily, some of the Sense 7 apps can be installed on Sense 6 without root. Here are the ones I've tested on my M8Sx (single-SIM, WWE Asia). Some of these apps are available on APKMirror and the rest are extracted from Sense 7 ROMs or system dumps of the M8, M9, or A9. They can be uninstalled cleanly - if you can't uninstall them from Settings, try dragging them from the app drawer to the trash icon, or using an application manager.
- Camera 7.30.581119 (23 Jul): Not much has changed apart from a few visual tweaks. The app seems to take photos a bit faster.
- Contacts 7.0.502006: This also includes the dialler with white buttons.
- HTC Mail 9.0.550299
- FM Radio 7.0.549341 (26 Jun): Fixed problem with application header.
- Calendar 7.01.591006 (6 Aug)
- Tasks 7.1.658473 (17 Nov)
- Sound Recorder 7.0.549341 (26 Jun): (Note: By default, recordings are stored in the phone's internal storage - if you've set it to Memory card before, you'll have to set it back again in the app.)
- Clock Widget 7.50.628268 (21 Nov): Two new clock widgets are available - a THEMABLE transparent one and a solid one. This version is taken from the new A9's system dump.
- Other widgets: Auto Rotate, Background Data, Bluetooth, Calendar, Contacts, Data Roaming, Data Controls, GPS, Hotspot, Mobile Network, Music, Power Controls, Profiles, Ringtones, Screen Brightness, Screen Timeout, Sync, Wi-Fi. I haven't got the time to test all of them, so please report if they don't work. Most of the widgets look the same - the only difference is that they now follow the new theme's colour scheme.
- Two small apps: Archive Viewer 7.0.623146 and Make more space 4.0.552526 if you want even more consistency.
- Personalise 7.1.631213: This app lets you download and apply fonts from the theme system of Sense 7. Tap "Font style" and then the plus icon to download fonts, but don't tap "Apply" - the app will tell you something is missing and try to download the font again. Instead, go back to the font list and select your font. Note: You won't be able to chose Sense 6 themes. Uninstall this version to get the old themes back.
Here are some images of the new apps: http://imgur.com/a/ZaI5u#0
In case you haven't noticed, some of the apps have square dots on the menu button and some have round dots. The ones with round dots are taken from MaximusHD ROM, based on Android 5.1. Perhaps on 5.1, Sense 7 will go with round dots like stock Android.