Compression ratio

Exhaust manifolds are a personal choice to run that looses power to headers. At your level, it’s not huge but a little noticeable. I wouldn’t worry a bit. There are good reasons run exhaust manifolds.

What size rim and tire?
The wheels are 14 inch, rear tires are 245-60R14's. It had headers when I bought it, but they were cheap ones and I had to cut them off.