"If you wanna go faster you should try my bigger cousin"
First half is a play on the old WTCC theme song, "Race faster". Why the WTCC theme has to do with this is to come.
The second half refers to the BMW M3 GTR Race Car, and M3 in general, which, no s-word Sherlock, is the bigger cousin of the regular 3-series.
"I'm the only of my kind in GT, though there used to be quite a few of us"
The BMW 320i is the only real touring car of it's category, though back in GT2 there were quite a few obtainable by race mods. Yes, maybe they were BTCC cars BUT the WTCC cars developed from ETCC which was basically a european BTCC. So how is the WTCC theme related? The 320i Touring Car was the first car to win the modern WTCC after it's revival in 2005 (descibed above)