Actually I am well versed with the JGTC and have been watching it this year quite alot...not on Speed btw.
The Skyline (which has now been replaced by the 350Z in the JGTC) actually ran a VQ33DET which is a destroked turbo version of the VQ35 found in the G35 and 350Z. The SR20DET's they run in GT300 class have EXTREME valvetrain modifications - by extreme i mean redrilling the head to accept a shim/bucket arrangement, using a CA18 camshaft and a ton of other stuff. There is basically nothing even close to stock about the SR valvetrain in those cars.
I don't believe the Skyline has ever run the RB26 in JGTC (but I could be mistaken), much like the Supra used the 4 cylinder 3SGTE for weight and balance. FYI, the Supra now uses a 5.3L V8 with a rear mounted transmission in JGTC. Comparing a near-stock RB26 with any JGTC engine is really not appropriate.