Check out the rx7.com fuel injector calculator here:
http://rx7.com/cgi-local/2ndgencalc.cgi
Keep in mind that the resulting numbers are BHP FLYWHEEL HP numbers.
Knock off 15% for most DynoJet numbers.
550 + 720 will get you maybe 250hp at the rear wheels.
I dunno where you can get just the GReddy IC pipes?
GReddy only sells full kits.
You might get lucky finding someone parting their kit though...
-Ted