It depends. If your horses are on good pastured, there is little point in feeding them more than a sliver, if that (except during winter, of course). If they are not on pasture, and they are not worked hard, and you have access to good grass hay (e.g., timothy), then a mix is "optimum". If they are worked hard, then nothing beats straight alfalfa.
I have never noticed that it makes horses "hot" (hyped up). I have heard a lot of people say this, but it is most likely due to a particular cutting , not "in general" - the protien content can change a lot from 1st to 4th cutting. But, if it does....and you can ride them "hot", just think...it can only get better!