Amazing.
Why wouldn't 2.2 sprites be used there?
Sonic's head definitely looks more like it does in 2.1 than 2.2, but it's actually been given a lovely touch up and so in effect is more like a blend of the two Sonics. Tails has a spriteset that is completely unique to Kart, while Knuckles and Metal Sonic have sprites based on their existing sprites—this is all as far as I can tell, anyway.