I would not use Red Cell unless the vet says to use it after a blood test. It is vary rare for a horse to be anemic. Fix your feeding routine first and see how he does. More evenly spaced forage and level out the grain rations and stick to one balanced ration. Excess iron in the diet is bad and depletes copper and zinc.
Just because this one person one time said TBs sometimes need blood builders means nothing at all. You have nothing to base a change off of yet except a guess and over the top of a poorly designed feeding routine. Fix that first. Its free.
Get a vet and do some basic blood tests first if the horse is truly exercise intolerant and not just unfit. Breathing heavy means nothing without something to base your finding on. What is the horse doing? He will be breathing heavy if he is unfit and working or working hard before his is warmed up properly. How much fitting up has he had? Is he being ridden regularly? Is the rider doing proper trot and canter sets to build endurance and wind? All these things matter. Otherwise, adding willy nilly is just wasting cash and possibly causing harm.
I submit that you need more information to make an educated decision at this time.
You have a lot of unknown factors listed, but they are all known to me. I don't know how much information you need, but this horse definitely has some behaviors that are causing me concern.
I have had two horses in the past that developed COPD. The first, a QH gelding, was found to be allergic to hay and had great difficulty with dusty conditions. I could have that horse's lungs built up in 3 weeks with interval training, and be able to ride normally by week 4 every spring after I gave him the winter off. (no cold toerance, in case you wondered) Eventually he had to have albuterol on a daily basis, which meant the end of his show career. He was the worst.
The second one was a grade QH gelding that I competed thru 1st level in recognized Dressage shows. He never did require medication because I was very, very careful to watch his environment to avoid & reduce triggers. I could canter that horse for an hour straight without difficulty in his prime.
So when I say this horse Drambuie, really worries me because he is breathing hard after a mile riding, it is because he is not responding to my tried-n-true interval training method of building up the lungs then the muscle and endurance. I can't even do trot/canter sets because he runs out of energy. I've only been able to canter him twice. Something is very wrong.
I can also see that you have a very low opinion of my feeding routine. Well, I have had numerous horses over the years from a shetand pony to a draft cross and many in-between. I have never had a problem with feeding up to 2 +/- quarts of feed on a daily basis in one feeding to an average sized horse. All the literature I have ever read says to stay under 3-4 quarts in a single feeding, which I do. If I had more pasture available, they would be outside longer, but I can't let my pasture turn into a barren wasteland by leaving them out 24/7.
The shetland had to be keep off pasture for most of the day to keep her healthy, and I put shavings in her feed to make her feel fuller without adding calories. That is an old timer pony feeding trick I learnt as a youngster.
The statement you made " this one person one time said TBs sometimes need blood builders means nothing at all" is not accurate. The lady that was explaining the issue to me is the co-owner of a very successful foxhunting stable. She has many horses in training and also breeds TB and TB crosses. I very much do respect her and the information, unfortunately I did not have a tape recorder or pen & paper with me at the time so I don't remember exactly what all she said, which is why I sent out a "feeler" to get feedback about the subject.
My hay is of good quality for the area. I can't improve on it with out driving at least up to Kentucky to get a load of timthy hay, which is not practical. My supplier delivers and stacks it for me too, which is a huge plus for me. The only way I am changing Drambuie's hay is if the vet says he is allergic to it. And I am already using slow feed bags for their evening hay.
I, like most other people, have to weigh my options and do what works best for my horse, me, and my money. Sometimes I have to make choices I don't like, but that's life.
Now, if you or anyone else has helpful advise, I would be more than happy to hear it. After all, I started this thread to help my horse by growing my knowledge based on other's first-hand experiences.