hspmick: Having a .63 a/r will allow less pressure in the exhaust manifold/exhaust, and less pressure there will allow for a better breathing exhaust. If your motor has little spool issues with a .63 a/r, like a B series motor does. Go for it. However with a D series your spool will be 2k rpm slower (putting you around 4500-5500rpm). A .63 a/r will most certainly allow for larger power gains alone.

On a side note, moving to a stage 3 wheel will flow nearly as much as a .63 a/r and keep the backpressure to a minimum. It's when he starts to reach the peak power of the turbo where he will run into problems with backpressure on the .48 a/r. Although i don't see that happening.

Hope this helps.