Same here. There's still so many tracks and variations I haven't tried yet in GT6. I'm more than content with only Bathurst and the Nordschleife to race on! :-)
You should change the weight (642kg) and go for about 700bhp (they use demo engines for these things, not the same engines that go to races). You're right about the long box, even for "heavy duty" demo engines there's no point spinning them too hard.