Ok first off you will not acheive any better performance from a supercharger than turbo charge on a standard turbo skyline.
second not all skylines are twin turbo only the GTR is twin turbo.
Now factory standard the single turbo skylines reach peak boost (max boost) on or before 3,000 rpms. The fact the cam profile etc are not designed to reach peak torque and hp until after 5,700 rpms for torque, 6,350 hp. So spending a shit load of money for a super charger which will gain you nothing seems a huge waste of time.
On my GTR which still runs factory turbos I've had the ecu remapped. I am running 24degs of advanced timing which is 8 more than factory, my turbos spool at 3,200 rpms yet the car does not make decent hp until nearly 4,000. Peak is 6,700 and peak torque is 5,800. I've just ran a grass gymkanna and did very well in it. I was quickest on all the courses beating out a couple of subbies and evo's and all the rwd cars.
NOS will definately help you but on a standard bottom end engine you are shortening the life of it dramaticlly.