Why is no one suggesting you start working on a good lighting setup?
Your 17-55 is a good lens for what you do, and the trackside stuff seems to work well for you as well.
This guy wants meticulous detail shots, at high apetures, so.. lighting seems to be the way he should be moving.
I'd rather see you pick up one of these bad boys..
http://www.paulcbuff.com/fsb.php
and an alienbee and a nice large reflector like this. http://www.amazon.com/Interfit-INT271-Reflector-35-Inchx70-Inch-Bracket/dp/B0024NKHA4/ref=sr_1_3?s=electronics&ie=UTF8&qid=1399997876&sr=1-3&keywords=Large+reflector
He could set this up on one side of hte car and reflector on the other, and get all the lighting he needs inside the car, while he's shooting.