the jaco lilac and yellow combo is probably the best combo on track right now. It works both in high tracktion major races as well as club races. Most companies went to the large wheel because they are lighter. less rubber make the entire wheel lighter. also most are not cutting them real small anymore. at the IIC the "magic" size was 43.5 for the rear.

I have run the "old" tires as well as the jaco lilac/yellow combo. the lilac/yellow combo is faster, but I am still competative with the old tires. with the new brushless 17.5, the purple/gray combo seam to push. you can run a gray rear, but the yellow actually frees the rear end up. also the biggest that I would run would be a 46.5. anything bigger will rub a well mounted body.