Have you tried conditioning your battery?.....I can't remember the thread for that, but you can search "Conditioning your battery". The thing with battery life is that it is a lot up to the user. If you charge your phone and just leave it...then your battery will last all day and then some. But, if you use it moderately to heavy, then maybe a day. You can always buy the REZOUND battery and put it in your Thunderbolt....yes, it does fit and that will give you better battery performance.
Also, Gingeritis is a great ROM that I have ran a few times. As far as tethering?, you can download "Barnicle Tether" and that should get you going.
The Kernal comes with the ROM itself. You can download separate Kernals (i.e. Ziggy), but I haven't. The latest radio out is Mr4. I don't know if Gingeristis is using that one, but you can flash to that one without a problem. I have Mr2, Mr3 and Mr4 on my phone and can flash to whatever one I want at anytime.
I believe Gingeritis is SENSE 3.0. I know Infected Eternity is SENSE 3.5.