GTR is another beer mate
It is turbo charged car and there the exhaust systems cost 3 times more than our cars (e.g. Akrapovic...). They are engineered superb! After replacing the standart GTR exhaust with Akrapovic one, YES a remap is needed, because the whole backpressure in the exhaust system is now differend than the stock one.The diameter of the pipes is bigger too, so the turbos need to be set for absolute new spool times etc. You can even increase slightly the boost because now the flow will be different, more acurate and easy!! But in our case, our college wants to replace the stock exhaust with a decat one in a NA car! The remap will cost more than the exhaust sytem probably, and in his case is not needed! In my race experience, for all my NA race cars we replace the standart exhaust with straight ones, and never do a remap just for this mod! In NA cars, the aftermarket exhaust will free some bhp, but if it is compelte from exhaust manifold to the back - Supersprint Ragazzon or some professionally made for rally purposes. The gain will be slight, the flow will be so much easier and the sound will be brutal. For a complete exhaust system you can go for remap, but you need to be sure that the exhaust system you buy cost twice the cost of the remap
This automatically made the ebay ones not an option!
So briefly, our college want to decat his car!!! What remap!! Noo, noo, just do this in order to free the flow of the engine. You will feel the difference in first 50kms, then everything will be normal again, because the gain is really really minor!!! This is for NA cars. They are always been a hell of expensive to tune or race. It seems we are now in the Turbo Era, where things happened for less money and the results are magnificent.
Regards and don't spend money for nothing