I also agree with FatBooty. I've had underdrive pulleys and they are worthless.
You could easily throw a nice T3/T4 turbo kit on there and SMOKE C5s with little effort. There's a local guy with a stock B18B in his civic hatch and a T3/T4 turbo running about 8 psi and Hondata. He runs an easy 13.2 1/4 with room to grow.
h:
To be 100% honest, the C5 is more about bragging rights to those that aren't very familiar with the different engines than anything - imho.
It's so much more worth while to have a well done B18B that is fast as crap than it is to have a $4k engine with a pretty red valve cover.
jmo