If the mechanic can read a code and not know what cylinder is causing the misfire, I would recommend a new mechanic. The code for a misfire is P030x, x being 1-6 depending on which cylinder is causing the misfire. If he doesn't know that, he needs a little more time in the classroom.

What was done in the full tune up? Not ever shop does the same thing during a tune up.

One of the things that can cause a misfire is a vacuum leak. The injectors can also cause a misfire, along with the plugs and wires and the distributor.

The misfire will decrease the gas mileage a little, but the big culprit is probably the Knock Sensor. A faulty knock sensor on a regular engine isn't a big deal, but on a Supercharged Engine it is a big deal.

I would take it to an Autozone or somewhere else that reads codes for free and find out exactly what codes the engine is throwing (the next time the SES light is on). Or buy a half decent code reader for like $60 - $70 and find out for yourself.

-Michael