Ahhh the good ol' A-series!
These are pretty good motors to build, because if you do it right, you can build a pretty torquey motor out of them. From what I've been told on Datsun1200.com, they can handle revs in stock form up to and not 1RPM over 6500, but I would never take mine higher then 6000. I've heard of gains through that kind of setup ranging from 10-15HP, but thats depending on a lot of variables.
If you want to rev higher, then you want to look for stronger valve springs like the GX springs. Also, if you haven't already you need to upgrade from the stock coil. Now that you're putting more air and fuel in the engine, you want to put more spark through to make sure you burn off every little bit of gas. More spark=more efficiency and more power. With that said, you also want to open the spark gap to take full advantage of the increased spark.
A good coil for the A15's is the MSD Blaster series. I'm running a Blaster 3 coil with the 8OHM ballast resistor (REQUIRED!!!) and I can tell the difference in the way it revs, idles, and how much cleaner the new plugs seem to be.
You might try jumping on the Datsun1200.com forums for this question, because those guys are A-series gurus.