With my '04, I get an average of 22-24 MPG driving a combo of city/hwy on winter gas. In the summer, it's more like 23-25 MPG. at least for me, winter blended gas isn't that bad. Are you really certain about your before numbers? Those are the best fuel econ numbers I've seen for a GP. There are a couple of threads/polls on this forum, and it seems like most get closer to 20 MPG. Driving habits I'm sure.
I know that the EGR valve sticking closed can hurt MPGs and cause knock. But I would think that should throw a code. Just double check that it's not loose or something. It's possible to bump it while doing plugs.
How many tanks of gas have you gone through? I wonder about a bad batch of gas. You could always fill up with premium and throw in a can of seafoam for good measure. Bad gas can cause both lower mileage and knocking.
I can't find the heat range on those plugs, but if they run hotter than stock, it could also be your problem. Looks like they should work though as I found people with Corvettes using them.
