I don't know much about bowed tendons, but as long as you are always doing regular check-ups with the vet, and the vet clears him for barrel racing, then obviously you're okay.
I use Professional's Choice SMB 3 boots on all four legs on my horses, plus bell boots on the front feet. They offer more 360 protection than splint boots do, and do offer a little bit of support. Not as much support/shock protection as the company claims, but the research does indicate that it does give a little.
Now you will get many different opinions on which name brand is best. It's a matter of personal preferance. The only suggestion I can make is to buy a name brand boot. Do not buy a cheap $20 pair of boots. You'll get what you pay for. They will be stiff and they will not mold nice to your horse's legs. Yes, the name brand is going to run you over $100 for all 4 boots, but they are worth it and the boots last for years and years if you take care of them.
∞•*˚ Βгįťţαňγ ˚*•∞
It is not enough to know how to ride; one must know how to fall.