I know the JWT ecu ups your rev limit to 7k, so my guess would be that is the safest you can go. As for HP add-on, get a cat-back exhaust, intake, headers, increase the compression, port + pollish, cams, JWT ecu etc. Remember not one product will yeild the most HP, they all work together to produce power, especially the header, intake and exhaust. We had a good thread about N/A tuning here:
http://www.zilvia.net/forums/cgi-bin/topic.cgi?forum=2&topic=955
That should help you out a lot.