You are 100% correct, I wont contest that. However, buyers who are looking for a modified GT-R, or have the intention of modifying one, will realise the value of obtaining a car with the majority of modifications already done. For the first time GT-R owner it might be hard to justify a 15-20k premium over a decent quality standard GT-R.
Those with the intentions of buying a standard car will look elsewhere, buy their GT-R and 2 or 3 weeks later realise that these cars are not meant to be left standard. They will then start to journey down the path of modifying it, and empty their pockets many times over. I was in this boat, and if I was to do it again I would definately look at purchasing a modified one. Would I import a modified car, condition unknown? probably not, as these cars are far from being cheap to maintain/repair. Would I buy one from an enthusiast locally with all modifications documented and performed by reputable workshops, for sure.
Re. the 350z suggestion, there are no locally delivered cars this side of 200k that are even in the same league as a 500+HP GT-R. If you, however, prefer the smell of a new car to performance, the 350z is a good option. This thread was about buying modified GT-Rs, and looks like it could turn into a meaningful discussion on used GT-R prices, hopefully it wont change.