The R31 rotors are a couple mm smaller in diameter than the Sigma, but the R31 rotors are vented and thicker. After doing some research last night, the R31 GTS rotors are larger than the standard R31 rotors, vented and just as thick. From what I can tell, the GTS struts are similar to the standard struts, so it might be around as easy to install them. Both the Sigma and R31 bolt spacing from the steering arm to the struts are identical, even the thread is the same. The Sigma steering arm is thicker than the R31, and the bolts are also longer, so the Sigma bolts would be needed. It might not be a huge upgrade to the standard R31 rotors, but if the GTS are the same, then it might be a good choice for an upgrade. I am not sure about the GTS version, but the standard version struts might be easier to source than Starion struts.
As for the strut top mounts, the R31 is 3 bolt, IIRC GE and GH are 3 bolt and GJ-GN are 4 bolt. I have not done the measurements yet, but it might be possible to use Sigma GJ-GN strut mounts on the R31 struts to make them fit. The height of the R31 struts are close to that of the Sigma struts. For the GH and maybe the GE, The R31's steering arm might bolt to the ball joint on the control arm.
GN steering arm bolted to a R31 strut
![Image](http://i620.photobucket.com/albums/tt288/Cottees/Sigma/DSC00392.jpg)
GN steering arm and control arm
![Image](http://i620.photobucket.com/albums/tt288/Cottees/Sigma/DSC00399_2.jpg)
GH steering arm and control arm
![Image](http://i620.photobucket.com/albums/tt288/Cottees/Sigma/DSC00401_2-1.jpg)
R31 strut assembly
![Image](http://wiki.r31skylineclub.com/images/f/f9/Suspfr.jpg)