From what I understand the touch keyboard on the Imagio is the same as the touch keyboard on the TP2. I too am a heavy texter but haven't had issue with the Imagio's touch keyboard aside from easily making smilies or other symbols. For this I might just install FingerKeyboard 2.1 (which I used on my Omnia and loved). With FK2 you can customize your keyboard and layouts (or download someone else's layouts). Before you make the hardware switch though why don't you install FingerKeyboard on your TP2 and force yourself to adapt to the touch keyboard for a week. Don't slide the TP2 keyboard out for any reason. That in itself should help you make the decision. Win6.5 is nothing more than a skinned Win6.1 with a few creature comfort changes. If you're using TF3D or SPB MS3 then you'll not notice much difference in the underlying OS.
Here's the FingerKeyboard link:
http://forum.xda-developers.com/showthread.php?t=501122