Hmm, I would likely race the set with the most FRESH set of tires. The Hankooks are amazing new, and drop off quickly with age. Thus, if one set is half the age of the other....then I would expect it to be a few seconds faster.
You are certainly right that the 225 on the 7.5 will be slightly more sloppy to steer then the 205 on the 7 inch but it should still be slightly faster if the tires are equal freshness. The exception "may" be if totally stock suspension (depending on surface grip and other variables). If suspension totally stock then the 225 may lean the car so much that it lifts a tire and the car plows rather hopelessly. So, assuming at least front sway bar upgrade and set of KONI Sport shocks I would want to run the 7.5 wide and 225 combo.