I have been shooting 3 under for 2 years now, and have read most of the posts on this forum about 3 under since I switched. None of the posts have mentioned any loss of performance from 3 under. That said, there probably is some loss of performance, as the nock point is generally higher, which puts it further away from the midpoint of the bow. There is sometimes an increase in noise, and I assume noise is energy not being used to propell the arrow.
It would be a fairly easy hypothesis to test, as most bows can be shot both split fingers and 3 under. Just tune the bow for the same arrow, and shoot that arrow both ways through a chronograph.
BTW, I doubt that the results of your test would persuade me to switch back, as I find 3 under to be a much more accurate way for me to shoot, with no loss of performance that is evident to me, at any rate.