There are subtle differences I've been noticing. I asked the identical question a few months ago.
I didnt get much answer, but I did a little bit of research into it.
They are all pretty much the same. The only JGTC that is significantly different is the Pennzoil Nismo. Its about 110lbs heavier and has slightly more power.
Other than that the rest are the same. There are some other little differences. For example, the Loctiti Zexel GT-R has about 10 ft-lbs more torque than the Calsonic. The Denso Supra also has about 7ft-lbs more than the Castrol Supra.
I am going to look inMK's for the answer. Oddly, thats something I just thought of now.
EDIT: After looking at MK's they are all the same from what I can tell.