As an early (December 03) M200 purchaser, I can't give you any direct comparisons as to speed and performance between the 1.5 Banias chip and the newer Dothan chips. I'm sure others can and I'm sure newer and faster will always be newer and faster.
I can say this in answer to your question. I run OneNote and most Tablet PC Apps that you read about and don't have any problems with speed and peformance on my older M200. That said, I have upgraded the memory to 1 gig and upgraded the hard drive to the Hitachi 7200rpm model and this keeps things purring along quite well. I'd recommend those upgrades even if there wasn't a faster chip out there.
If the price drop was substantial enough on the 1.5 and that gave you the room to add some memory and a faster hard drive, you would build a tablet that can handle current tablet apps on the market with no problems. There's my $.02.