If you wanna know just the power etc, just go for a dyno day and do a few power runs. Dyno tuning is only useful if you can tune the ecu (as the name suggested), or if you can remap your ecu.
You're best to ask someone that's got knowledge on the 35gtr if you do own 1 as not many people know much about the mods available for the 35gtr. I would assume most ecu we're using (other gtrs) would not be compatible.
Also, ask yourself why are you doing this? Is the car not fast enough for you? Are you planning to track it? If not, I wouldnt bother touching it since they're an awesome car from factory.