I would stay away from the Aisin 997 gearbox currently. THEY ARE NOT BULLETPROOF !!!
They use a fiber lined, yes FIBER lined synchronizer ring that will not keep it's coefficient of friction as heat develops in the box. Guys who are running the Koni challenge are having large problems keeping these in service, with Porsche currently only allowing replacement gearboxes to be sold and no parts released directly to date. We have developed some parts for the geabox, but it takes time and large financial resources. We have 2 currently in the shop for repairs and fitment of new differentials, and some of our new parts. I am not impressed with the syncrho issue, and I wold say it would be a better option to use an inverted 996 C2, or Boxster 5 6/S, even the Cayman 5/6 speed before looking at the new 997, at least you can get parts for those. We have had extensive experience with the 996 gearbox, and it is a stout unit that will readily hold the torque of a well prepared street car, around 400-500 ft lbs. in my estimation. Check my website home page for a red 996 we did for an Ultima in the UK. The inversion is a little tricky, and we do a good amount of machining to the gearbox to correct some issues we find with the manufacturing, but they are a fairly reasonably priced option. Lsd's and Tbd's are available as well. Light too.