Yes, they are basically all the same. Some are a little better than others due to the ingredients, but as a whole, given how little you feed, even a bit lower quality ingredient doesn't make a big difference.
That said, they aren't all created equal in terms of palatability, and yes, some horses don't do as well on some as on others, even if it's a higher quality RB, like TC30.
______________________________ The CoTH CYA - please consult w/your veterinarian under any and all circumstances. - ET