What I can recommend by experience is Geoff from AMD (AutoMotive Developments), he did just a remapping on my 964 C2 and brought it from 248bhp to 272bhp. I had lost 2bhp from the factory over the years, but I got +24bhp has with the remapping. Also Geoff managed to improve a lot the torque, giving more power at less revs, that is what really makes the car feel quicker and not the peak power.
But if you want to go for the ultimate low intrusion (and thus less risky) modification to increase the power, go for Geoff's Hotfilm. The idea is very simple, to remove mechanical parts from the air intake and replace them with electronics (hot-film) to allow the engine to breath better and without the timelag of mechanical movements. This system is present on standard 993s but Geoff has adapted it for 964s. I believe you can go over 275bhp easily with this system. (Geoff's has a red 964 himself). It sounds like if I have commercial interest on AMD, but I'm just a happy customer. Also, I'm sure that there are some other good people tuning 964s but I just happen to know this one. Cheers, Manuel A