When I did IMFL in 2014 I wore a vest made out of a 10 gal kitchen garbage bag under by kit. No aero penalty, blocks the wind and keeps your core warm. If you get hot you just rip it off and throw it at an aid station.... Toe warmers, gloves and a skull cap are also pretty quick to put on in T1. You can also tape the helmet vents. For a sprint I would consider keeping the wetsuit on :)