If you're looking at a v8 R8, look at the Stasis toots blower. If at the V10, VF Engineering has a bolt-on roots blower kit, GMG racing, AMS , and of course underground racing, have turbo kits. IMO, go with the 997tt if you're looking for the more performance oriented car. However, the 997.2 has it's limits.....the limits of the PDK, DI fuel system, and stocks rods are being reached with a simple tune and exhaust. As for the 997.1, it will be an expensive car to get power from.....you'll need an upgraded clutch (if you decide for a 6mt), brv's, tune, and full exhaust to make 550whp to 580whp and it will cost much more ($7500-10,000) compared to a GT-R (which you decided against already) which will cost around $3000-4000 to make the same power. One you decide to go to upgraded vtg's, the costs quickly accumulate......definitely not cheap cars to mod.
Not sure if you're considering a sedan, but the w212 e63's and cls63's with m157 are making amazing power with very few mods. Weistec and AMS have already made bolt-on turbo kits making 720whp and 780whp respectively. Weistec uses stick frame turbochargers and their upgraded turbos cost around $8500......a "bargain" for the kind of power those cars are making.
Jet Black / Gloss Black Grilles / 8000k D1S low beam / 6000k HID fogs / GP Thunder 8500k angels & cornering bulbs / black reflectors / gloss black vinyled window trim / tints / smoked rear turn signals / black roundels / AC Schnitzer roof spoiler / VRS style decklid spoiler / Matte black Stripes / Rear fogs / matte mirrors / 19" LM reps / APR lip