There are a couple of Single Turbo GTR's here in New Zealand.
From what I have seen there is no difference either way. I have seen a R34 fitted with both. A T88 Single or 2 slightly smaller ones. Power was the same, just a few different characteristics.
The main reason for a single turbo setup is it is cheaper than modifying and buying 2 turbos to achieve the same thing. The GTR is ofcourse twin turbo from the factory, but when much larger turbo's arw used the exaust manifold has to be replaced.. therefore a larger one is recommended.
On a 6cyl car such as a GTR. They say for anything over 400HP single will work out the best.