btw, i dont think this is an unrealistic question. i understand you are the moderator here, but my receommendation would to be to use your moderating capacity in the interest of the forum.
why would you NOT think there would be a difference between the wifi capacity on 2 different carriers, on 2 different devices?
Correct me if I am wrong but are they not both made by the same manufacturer. TMO provides UMA ATT oes not. If I recall correctly WiFi won't make much difference as the processor in the BB's is the biggest hold back for WiFi making much difference. I have tried it and ill stick with edge for now.
I will let you know this week; I am going to try my unlocked 8320 on ATT because my signal for Tmo is horrible in the office and I think they have a repeater for ATT. I will check the wifi, but I can almost promise you Wifi will work fine with an ATT SIM in it. Rather, it is UMA that will not work.
I think you will find the wifi on both phones is the same as I woulf guess they use the same wifi hardware drivers. Regardless, you are not going to see a huge difference. If you weren't happy with the 8320's wifi, you probably won't be happy with the 8820's.