SWIM has tried impure mescaline extracted from San Pedro with pure LSA. The LSA overpowered the trip, and it was only about 500 micrograms (if I recall correctly), but came on like 5 mg. Something in San Pedro amplified the effects of the LSA about 10 times.
The experience has no elements of a mescaline trip at all. The mescaline was totally drowned in the LSA effects. However, the LSA sedative effects were lost, and the LSA became very visual. But the visuals were not like mescaline at all, they were like LSD visuals.
SWIM cannot get LSD anymore, so he can't comment on LSD + mescaline, but I imagine it would be sort of similar with the LSD dominating the experience.