I brought Autech into this as I thought you were concerned about the quality of replication due to the car being a conversion.
....after all that, the weight of your argument which justifies the reduction of the asking price by $40-50K is the fact that you would have to list all the mods on your insurance policy (which you probably already do for your current vehicle)........and that the vehicle would be listed as a GTS4 instead of a GTR (which might even save you some money on your premium.....god knows I'm paying more per year on my GTR then I ever was on my old GTST).......
THAT is your justification???
I know your concerned about the re-badging and calling it something it's not but trust me when I say (with exception of the sunroof and VIN) this thing is about as much a GTS4 as what's sitting in my garage right now.
Also, I don't know why your talking about engineering (I assume you mean VSCCS compliance).
The vehicle was personally imported and complied with the mods already done (Yellow Tag). The vehicle is VSCCS compliant.
.....I don't recall Autech actually selling any vehicle through Nissan that had what you call "huge power modifications" either. Even with the radical design changes of some, they were still lightly tuned. They still had to suit the requirements of the JDM after all.