This current thread might be of interest regarding beefing up the auto tranny:
http://nsxprime.com/forums/showthread.php?t=102734
Obviously the NSX is also about driving position, looks, handling, sound etc - BUT you mention you want to add power - so it seems that straight line speed *is* important to you - so I just want to lay it out:
Auto NSXs are worth a lot less because they are slower than Camry, Commodore, Falcon, and you can't do much about it. The *only* advantage of the auto NSX is that it is slightly less taxing in city driving.
In 0-100km/h terms the manual NSX is quicker than all but the fire breathing V8s, and with ~$7,000 of mods can be at least on par with those. You get the much touted crisp gearbox, with only the fingertips being needed for gearchanges. You get to blip the throttle on downshifts. You get shorter gearing, and a direct drivetrain link between the engine and the road for immediate throttle response. You also have no power steering - and unless you are a valet driver, this is a wonderful thing.
On balance the Auto NSX is still a great car and if that is your choice, at least knowing the above it is an informed choice that you won't regret.
Post up some pics if you pull the trigger!