I would like to throw out the idea that once your horse reaches level 50 it stays there until you hit VET1, at which point you can feed him again all the way to the current highest VR rank. I think feeding 12 more times is hardly a game breaker and certainly should be something VETs get to do, if Zenimax wants to loose the 250g/day money sink and of course since VET levels (unless grinded take days) perhaps they could make feeding a VET horse cost 500g until VR5 then 1000g.
Of course I would rather they just raise the horse level cap to 100, if top speed is 75, why put a bar that looks like 100 is the cap? Personally I think a horse shouldn't level, I think it should just be fed and gain. If they want they could make if you didn't feed it for 2 days it looses the last point it gained, that would keep up the money sync but let the dedicated have 100 speed horse if they want to make sure its fed.