I have used the Enrich Plus (and Enrich 32) on my own horses and we have used it for client horses at the equine hospital. We saw very good results with it.
The issue with fixed formula or not fixed formula is a "grey area". If you have a colic prone horse than a fixed formula is a better choice. If you don't, then I personally prefer Purina's strategy of altering the formula slightly as necessary to maintain appropriate nutrient levels. With fixed formula feeds, the nutrient levels can fluctuate because no two batches of any feedstuff (grains, forages, etc) are exactly the same nutrient-wise because growing conditions affect the nutrient content. So, with a fixed formula feed the actual nutrient content can vary quite a bit from bag to bag though the feedstuffs combined to make the feed are kept exactly the same. (We had this same discussion in my university equine nutrition course.) Purina makes small alterations to the composition of it's diet to compensate for changes in the nutrient content of the feedstuffs from batch to batch to ensure the total nutrient content of the diet.
Licensed Veterinary Technician