I wouldn't call Litchfields or SVM a back-street tuner and they certainly know a lot more than Nissan dealerships about the GTR as 99.9% of their customers will be in Micras/Almeras.
I made my 350z as good as it could be, in my eyes, without spending silly money on it (well the missus would say it was silly money). I probably spent around 70% of what I actually paid for the car 2 years ago on modifying it. You can see around a 25-30% increase in power in a GTR for similar money as you would have to put into a 350z for only a 10% increase. I'll definitely be modifying my next car as it's a disease, it'll cost a lot less than £100k and I can't think of another car for around £100k that I'd actually want for performance/practicality.