Well I have seen more than a few 12A's making upwards of 500 whp, but I would consider them street cars. I'm not saying you can't make power on the stock ports, but a good port job will most likely be in your best interest. Look here for some porting options.
http://www.rx7forums.com/forum/showt...=7080#post7080.
325-350 shouldn't be a problem to reach. Remember to pay attention to other aspects of the car as well as the motor. Making power is good, but if you have crappy suspension, tires & no attention to driveline mods you are just hurting yourself.
I don't recall you mentioning what the purpose of this car is. Street or track?
A good all around turbo would be a GT35r, you could make the power you want & still have room to move up. On a S4 or S5 you could get away with using a modified stock turbo like one of the BNR staged upgrades, if going with the REW I know of a guy that make 405 rwhp on the stock sequential turbos. You are going to need to bump the fuel system in favor of some better flowing items (pump, injectors, ect.) Add in a good ECU with a good tune & you should be fine