hpqoeuit'd be mildly worrying if you got more frames using opengl in tf2, since as far as i am aware valve uses a wrapper for their source engine games (with the exception of dota 2 being on source 2) to transform dx calls into opengl ones.
I guess even if it's converting the calls there must be some frame benefit to the opengl version of some of these calls that are being made. My guess is it does intensely come down to individual hardware setup.