I might be wrong but I believe that HSPA+42 versus HSPA+21 has just as much to do with the carrier as it does with the device. So, in other words, if my device supports HSPA+42 but the network only supports HSPA+21, I will only be able to operate at HSPA+21 speeds. Likewise, if my device only supports HSPA+21, that's my maximum regardless if I'm on an HSPA+42 network.
That said, the AT&T Galaxy Note appears to have the hardware specs to support HSPA+42 (qualcomm dual core processor and band IV) but because AT&T's network only supports HSPA+21, they advertise the phone as an HSPA+21 device and that's all it will get on their network.
The difference between T-Mobile and AT&T with regard to band IV is: AT&T uses band IV for LTE and HSPA+ (when LTE is not available). T-Mobile uses band IV for HSPA+42 (they currently don't have LTE on their network).
So, if we can get a t-mobile modem that supports HSPA+ on band IV, in theory, we should get HSPA+42 on t-mobile's network since the AT&T Galaxy Note has the specs to support it. This is why people are flashing t-mobile modems now to get increased speeds but the speeds are not consistent or stable. I've gotten results ranging from 1mbps to over 18mbps on the downlink and .1mbps to 3.4mbps on the uplink (it's not consistent at all). This is probably because the flashed modem is not made for the Galaxy Note but was hacked and ported from another t-mobile device (like the Galaxy S Blaze 4G for example). When the T-mobile version of the Galaxy Note comes out, we should be able to dump and port the modem from the t-mobile version to the at&t version (since they should have the same specs - we hope!!) and ultimately give us more consistent HSPA+42 speeds......at least theoretically.