Any internet sharring .cab that is avaiable on the site is compatiable with the Bell ROM...As for the tweaks you are referring to its limited to what you can really do, reg edits are where most of your tweaks will come from, these come in .cab form (most of them) but they can be cooked into custom ROM's and this will make everything so much easier.
But there is a (super).cab floating around here some where with alot of the tweaks that help the performance on your device...if I find it I'll link you!!!
http://forum.ppcgeeks.com/showthread...ght=super+.cab