I think AnAmericanMother knows something of t-breds from the track. I know some people who are happy with theirs, after some initial training. They don't come off the track knowing anything but how to run, so you have to either be skilled, or willing to spend some money with a trainer who can basically start over with their saddle work.
I would add another wash stall at the opposite end of the barn and remove one box stall (there is always a line at our barn, and we have two wash stalls and 24 horses!)
Individual tack rooms are a great idea. I wish I had a dollar for every item of mine that's been "borrowed" over the years, despite my having my name stamped and stenciled on every equine item I own!
I have always been suspicious of overhead hay lofts ever since a fire started in one at the main barn at Rockridge Farm here in Atlanta many years ago. 34 dead horses. No ignition source - just improperly dried hay. And concrete block construction won't solve the problem because there's plenty of fuel from the hay itself. I think that hay and shavings or straw should always be stored in a separate building some distance from the barn.
One solution is the "Vermont barn" - built into the side of the hill so that you can wheel the hay into the upper story from a separate storage building on the uphill side. If the barn were built with a very sturdy frame, you could even tow the hay in behind a small tractor and toss it down.