I've heard and driven on the "shorter" IOM TT circuit before, in GPL as well as on its rFactor/GSC2k13(and AMS) conversions. Now the full 37.5 mile circuit has been released.
There are also some CTD fixes and lite textures/other stuff over at Jim's website.
Given how much fun I had on the shorter version, my personal track record with open wheelers being somewhere around 7:08 with the AMS Formula Classic 1988 and with "normal" cars being 8:30 with the Audi Ur-Quattro S1 in rFactor, I'm definitely looking forward to finally trying this track out.
Given that it took Jim alledgedly over 10 years to develop this track, I'm convinced it's more than worth looking at and enjoying.