So the root cause of your problem is wap.voicestream.com. It should never be used, as it is not supposed to support data. They block all traffic to anything other than the wap gateway, but their block is broken in some areas (like yours) where data gets through 5-25% of the time.
I'm sure that wap.voicestream.com is the root cause of your traffic issues.
Try blackberry.voicestream.com (the blackberry-specific Tmobile apn), or try internet3.voicestream.com (the encrypted apn for corporate accounts). One of these should work.
If you still have issues, call Tmobile, ask to be transferred to level3 data support, and tell them you need the proper apn configuration for tethering to the blackberry. They may be able to reprovision your internet2.voicestream.com connection too. The techs at level3 support are awesome - the best I have ever encountered in a large consumer tech company.
Also, The biggest loss of a downgrade is to app 8.302 is that the FM transmitter will no longer transmit spoken directions.