It really depends on size of the wheels and what tires you get. I had a 2006 HCH and replace the wheels and tires with 17X8 OZ Superleggera and 225/45/17 Kumho ASX tires. They really hurt the MPG on the car. I did mostly stop and go traffic on the highway and was lucky if I got 42 MPG. I suspect those are pretty good numbers with aftermarket wheels and tires. But, I think not getting low rolling resistance tires hurt the MPG. If you do switch the wheels and tires, look into low rolling resistance tires. The stock tires are and I am sure that will help the MPG if you do change them.

The car did look a lot better and the handling improved quite a bit.

I got the Si 17" wheels which came with Yokohama AVS ES100s. I'm going to try the Kumho's next because the Yoko's are really awful. They're extremely noisy at low speeds the rolling resistance has to be poor.

I still don't get peole that buy hybrids only to mod them at the expense of FE?

For me and I suspect others, it was a trading-off some FE for looks (personal preference and vanity). Still getting relatively good FE in a good, reliable car that's also practical. One reason why hybrids have been the rage vs. plug in battery for 100 miles.

One reason for the wheel/tire posts is to offer the effects of various tires/wheels against FE so others can make choices best for them.

I agree but then these same people (not all, but it has happened here on this forum) complain about their FE. Then people start questioning what they are doing, and it turns out it's their wheels at the root of the problem. Tires and wheels are probably the most important item on your car, which seems to be missed.