Paul & Don Vermillion;

Q: HP difference between NA & Turbo?
The HP difference between NA and turbo rotors is 3-5% approaching the torque peak.
See the bible, 81 edition (near the bottom of the web page), p.49.
    http://www.wankel.net/~krwright/cars/rx7/manuals.html
Here is a chart of rotor information:
        http://www.mazdatrix.com/faq/rotorwgt.htm
This ignores the intake port timing and size differences of the housings.
    http://www.yawpower.com/dectech.html

All intake/exhaust ports can be ported for more power at the top end.  Mild street port and extended street port are good options.  Peripheral port (PPorts) are a more involved option.
Here are a few web sites with pics of various port types - don't believe everything that is written - particularly idle RPMs.
    http://www.turborx7.com/portingpictures.htm
    http://www.mazdarotary.net/porting.htm

There is lots of information on these sites:
    http://www.rx7club.com
    http://www.nopistons.com

One of the nice things about the 13B is that most parts can be mixed and matched.  Turbo and NA have different size oil pumps.
You should not mix NA and turbo rotors.  Counterweights should match the generation of the rotors.  Looks for threads on balancing if you need to change a rotor.

Later model generations of engines have incremental improvements.
There are differences between the generations of housings:
    http://www.rotaryresurrection.com/2ndgen/tech_housing_differs.html

NA housings have an exhaust port noise splitter/diffuser in the insert. The insert can be replaced or the splitter ground out (with some difficulty).  (see bottom left of photo).  As has been mentioned, the NA exhaust splitter is worth 7-8db of noise reduction for an unknown reduction in HP.
There are also some minor passage differences between the generations...
Series 5 and later housings have a knock sensor above the top trailing spark plug (top right in photo)

Cheers
Cary