I've been experimenting with this the last few grows.
Long story short, I now very happy with the results using : 4weeks MH - 2/3 weeks HPS then back to MH to finish.
My buds seem denser, my stretch is lower, and i have probably lost a little in yield, but the amount is negligible.
I can't comment on the psychoactivity as I haven't ever done a side by side.. But what I can say, Is the last grow I did using this method; the weed has got me close to whiteying a couple of times now... Something I haven't experienced in a while .
I have been wanting to try a MH grow, I like your idea.
I might give that a go this time round and see for myself mate.