As mentioned in the MTC 2015 thread, I now have a separate set of rims for the track.
They are 'Linea Sport Super Spoke', rear 17x9 +38 and front 17x8 +29.
I also got some Federal 595 RS-R tyres from Gorilla Industries, 255/40 R17 and 215/40 R17:
My intention is to keep semi-slicks on these 17 inch rims for the track only, and keep normal road tyres on the 18 inch rims for the street.
EDIT: I know that Federal 595 RS-R aren't true semi-slicks, but they should be a good stepping-stone towards a true semi-slick in the future. One reason I avoided a true semi-slick (for the time being) is because they are generally much more expensive.