anybody know of some high quality spark plugs to improve performance?
This is extremely correct. One tick hotter won't hurt, but there's no real gain in any plug on a stock level.To hot of a plug NO GOOD
To cold of a plug NO GOOD
Unless you do a whole lot of modifications you need to stick with the plugs that came in your engine.
They are the correct plugs and as long as they do the job and ignite the fuel that is all you need.
No matter what advertisements you see you should know that changing spark plugs will not boost the horsepower on your Mustang. It won't boost the torque either.