Probably the biggest restriction on a stock GT-R, and certainly one that will make a noticeable difference is the air filter and intake set up. On a standard car, the power tails off very quickly above 6000 rpm. One look at the stock air box and intake plumbing will tell you why...
A very simple solution is to either buy or have made a 'top hat' adaptor, to mount a cone type or equivalent filter directly to the air flow meter. The picture below shows my first set up, using a HKS air filter.
This set up works well, however it can be improved further by ensuring that the filter is fed with cold air from outside the engine bay, rather than the hot under bonnet air. In order to this, I firstly removed the front fog lamp and mounting bracket, and replaced this with a cold air feed. The kit used was a 100mm cold air feed kit, available from Pipercross. This uses an aluminium trumpet which connects to a 100mm bore flexible duct hose. I cut a hole in the front panel directly behind where the fog lamp used to sit, and used this to allow the new cold air feed pipe to pass. To mount the trumpet in the bumper, I used a plastic drain pipe fitting, along with some screws covered by short lengths of 6mm bore hose to make the installation much neater.
At the air flow meter, I used a 90mm, 90 degree Samco silicon hose, so that now the intake air ponts directly down into the inner wing where the cold air feed comes through. I then used a K + N cotton gauze filter mounted on the end of this to filter the incoming air, and also added a guard to stop any stones or debris flying directly at the filter and puncturing it, whilst still allowing plenty of air to flow through.
The final step was to make a cover over the whole inner wing area, to ensure that only cold air could reach the filter. I carried out before and after tests using a temperature probe mounted by the air filter, and found that the average intake air temperature has been reduced by around 30 degrees C! Previously, the filter had been drawing in hot air from under the bonnet, in the region of 50 - 60C on a typical 25 - 30C day. This meant that if the car was driven slowly in traffic, or sat standing, it felt very sluggish until some speed had been built up to get cold air into the filter. With the new set up, this is no longer an issue, and the intake air temperature is always the same as the ambient temperature, ebsuring maximum performance all the time.