It's unfortunate that PD doesn't/didn't have the foresight to include those real world karting tracks in the game. I mean, they're already there to begin with... You'd think with karting finally making an appearance in GT they'd take advantage of things like that.
Of course I'm sure they're not modelled with a very high polygon count and have poor textures, but it should be very simple to spruce them up and include them as separate karting tracks. It really depresses me quite often how PD has so many great opportunities and doesn't capitalize on any of them.
Same thing with not including the Bugatti circuit at Le Mans... Why not include that track as well? It uses a good section of the normal course and you're already there taking photos and information, so why not add another variation that you're going to model (at least to some extent) anyway?
But anyway, back to karting, the lack of non-shifter karts and the presence of only one actual karting track (DLC, mind you) makes karting very boring in GT5. I know there are course maker tracks as well but frankly those are very bland and just not exciting at all to drive on. If you're going to include karting, please make it at least relatively interesting...