I have not feed straight Amplify, but do feed Omlene 400 [formerly Complete Advantage, for years]which has amplify in it.
Just like with people every couple of years there is a new diet trend for horses. And just like people they are tried for awhile with no huge differences and that quickly people move on.
As far as the fat question- OCD is the most well know risk
http://www.petersonsmith.com/vetnote...rses%20CKC.pdf