I have a set of Kumho KU31,s fitted to my S1, no problems at all with them so far on road or track (Hethel) and only �240 for a full set too 
http://www.camskill.co.uk/products.php?plid=m4b238s16p0
marc.
I have just fitted some Toyo TR1s to the rear of my Exige S1. I only use it on the road and they are not as good as the original Yokos in the dry. There is not much in it, the back is not as precise or as responsive (but only by a small margin). Also the tyres that came off were right down to the legal limit so this may make the old tyres feel crisper. That said the Toyos are about a third of the price of the original Yokos and should be better in the wet and last longer. As long as I get 12k road miles I will stick with them.