Well i am still finding issues myself with white levels, i use Auto white level still as on the 40d they are pretty good.
From memory im currently using spot metering, however with a dark coloured car you get a over exposed looking background and i find with partial metering the opposite can happen.
I normally flick between the two depending on the cars/backlight conditions.
I cant remember what i have been using most recently as camera is still away getting new shutter and i just use whatever works at the time.
Experimentation is the key. There is a reason ive taken over 30,000 photos in about 5 months and only have about 7,000 on my smugmug.
BTW, looks like the lense is working great for you.