I just finished a 3000 mile road trip where I looked at IATs in various conditions with the Helix intercooler. Temperatures were very stable. At steady-state, off-boost highway driving, the temperatures averaged 12 degrees over ambient. When I did a series of on-boost pulls, the temperature dropped 3 degrees and stayed consistent until I went back to steady-state driving. The temp would then temporarily raise 5-6 degrees for 30 to 60 seconds, and then stabilize at about 12 over ambient. After hard, on-boost driving, if I came to a stoplight and waited for 60 seconds or more, the temperature would raise to 22-26 degrees over ambent. By the time I was through the intersection, temps drop 5 degrees. In light-to-light driving with heavy traffic, the temps varied from 15 to 18 over ambient, dropping by 5-8 degrees during a on-boost pull.
Even with abusive, on-boost city driving, I could never get sustained temps more than 20 degrees over ambient.