It's been a good while since I touched proth_sieve now, but I believe there are a few internal parameters that could be tweaked to provide better performance for larger n ranges.

I agree with the people who said now is not the time to start sieving above 20M though. Just seems a bit premature and "pointless" right now. PRPing will be there in what, 5-10 years? 2014? Are flying cars finally commonplace then? Not to mention the possibility of a new algorithmic idea being discovered.