[Q] Modem question topic

Ok, so I have the AT&T i747 and I unlocked it the other day to work on the T-Mobile network. I am interested in assisting with Beta testing on a 4.3 ROM for the i747 and one of the prerequisites is to flash the 4.3 modem. My question is, do I flash the 4.3 modem for the i747 or for the T-Mobile equivalent? Also, if you wouldn't mind suggesting some other things to try as far as getting a better network and wifi signal, that would be awesome. I've tried several of the "wifi enhancement" apps and the "network enhancement" apps as well as going in and verifying the apns are set for the T-Mobile network (fast.t-mobile.com), etc.