copper core plugs will not give you less gas mileage
these boosted cars, just like copper cores better. I run them in my 99% stock GTP and I see zero difference in gas mileage.
As for you thermo. Get a new one (195* stock replacement) and see if it changes, If not your out 8 bucks, no big deal. Then start to look at the temp sensor.
As for your boost problem. Check your vacuum lines and see if any are broken or have come off of where they connect too. A broken line will cut your boost because it can leak out.
