The 2" pipes are big enough for a mild engine, unless you want to switch to headers. Most of your restriction is in the exhaust manifold, so larger pipes won't make much difference. Use low restriction mufflers and an H pipe to get the most you can out of the 2" pipes.
You could also use 2 1/2" Corvette rams horn manifolds and a larger exhaust, but I'm not sure if the routing (straight down) will work with your car. Dorman sells them as a standard reproduction part, so they are easy to find and not that expensive.