One of the most restrictive parts on a stock GT-R is the exhaust. Although it's 'upgraded' over that fitted to the GT-X, the differences are very minimal. The bore size of the pipe is very small, and is reduced further still at the catalyst. Don't be fooled by the tail pipe size either, that's the largest bore section, all the other parts are substantially smaller!
Availability of upgraded exhausts for the GT-R is very poor, the only ones I have ever seen are HKS, Scorpion, OMP and 5Zigen, and some of these may no longer be in production. The HKS is a 3 box, cat back design, the front section being 50mm and the rear section being 60mm. Whilst it's better than the standard one, it's still not a great exhaust, although if noise is of any concern to you, the HKS is very quiet. The OMP is a single box, 60mm exhaust and goes from the stock turbo elbow back. This exhaust is much louder than the HKS, and flows slightly better. The Scorpion is a 2.5 inch bore from turbo elbow back, I believe it's a single box design and loud! Lastly, the 5Zigen is a quiet 3 box system, with a cat replacement pipe option. As such, the bore at the front is only ~50mm, then 60mm further back. It also retains the stock front pipe, as does the HKS exhaust.
This is an HKS exhaust off the car.
The remaining alternative is to have a system custom made. Depending on your plans, 2.5 inch is fine up to ~300BHP, above this a 3 inch bore system will be better, however in the UK there are very few companies who will build 3 inch exhaust, Blue Flame being one of them.
My own car arrived from Japan with a HKS exhaust already fitted. To improve this, without spending too much cash, I replaced the front part with a home made section of 60mm tubing, so that the exhaust was now 60mm from the stock turbo elbow back. This made a noticeable difference to the perfromance of the car. At a later stage, I decided to replace the standard cast turbo elbow with a much improved 4 inch tapering to 3 inch bore section. In order to do this, the stock air conditioning set up was completely removed, including the pump and mounting brackets. This then left enough room to build a replacement part with a 3 inch down pipe, which I mated to some 3 inch flex tubing, then reduced down to 60mm to match the rear exhaust section.
Here are some shots of the dump pipe and front section before fitting on the car.
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Jaustech in Australia can supply the dump pipe / down pipe pictured below
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The pipe comes as too seperate sections to ease fitting
Here are some more pictures of a home made dump pipe.
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Since these pictures were taken, I have now had a custom 3 inch exhaust system made by Blue Flame Engineering, which mates to my original dump pipe. The quality is excellent however the combination of a 3 inch dump pipe and exhaust means that the boost is now very difficult to control, so I would not recommend this on a standard car. As a result, I have had to modify the wastegate on the turbo to retain some boost control!