Steven,
I believe I may fall into the catagory as to what answer you may be looking for BUT I just got a Matrix. The Xb has a 1.5 compared to the Matrix's 1.8 but I did some of the things you mentioned. I have brought partial hypermiling driving into effect as well as I bumped up the tire pressure up to 40lbs. So far over 829 miles I have gotten an average of 34.5 MPG. I've done more city driving with a lot of idling with the A/C on too. EPA ratings for the Matrix are 28city/33hwy so I know you can easily come close to consistently breaking 40mpg with the Xb.

I can see exactly why you would want to chose the Xb as it is substantialy cheaper than the HCH. I was actually looking for a used Xb but I had a set price of $10k I wanted to spend on a used car so I settled for the Matrix which I do not regret one bit. Being a "wagon" I have been able to haul much more than I would of ever carried in the HCH so it has suited my needs excellent so far. On average now I have taken a hit of 12-15mpg which I am not too happy about but it has evened out as the Matrix has suited my needs for hauling and being a cheap-er car.

AL

'00 Insight "Wazabi" -75,000+ miles
'03 Toyota Matrix - 81,000+ miles
'03 HCH-CVT-OSBM-41,000+ miles (retired 04-10-06)
"I'm very secure with my little Insight, no Prius envy here."