Realtime runs 17's for a few reasons...1st SVWC touring mandates them...although they gave the ITR's a break for 2 years.
2nd...there are more choices of tires sizes in 17 in Toyo spec T1s
3rd.....more stability on high speed tracks and sometimes more gearing.

But if it were solely their choice...as it was since the beginning....15 is the way to go.