Has anyone made a study of how much "self bias" affects response at 7.5ips?

Dolby has. This is why they invented (or maybe just named) Dolby HX.
It compensates for self-bias by reducing the bias when recording
something with a lot of high frequency content.

Correct. (Banging O discovered the effect, by the way.) But I was thinking of
speeds that would be considered "slow" by professional standards. (When I
recorded live, I rarely ran at 15ips. It used too much expensive tape.)