Don't forget about optional wheel sizes. For the s550's 20x9 wheels are optional for the base models, which means you could run a 19x9 in FS. I figure the late s197's likely have a similar big wheel option.
If your car is not fully prepared for FS, why would you even consider STP? If you do it for tires only, you will be more underprepared than you are now.
But then again, a V6 auto is never going to be super competitive in either class. So do what you want and have fun.