I don't think so. If agi builds were better with heavy horses, people would have it figured out long time ago.
It comes down to playstyle mostly. STR doesn't make your horse better, but AGI does. I still think the most important stat for a horseman is riding. Cavalry differs from infantry in philosophy. Infantry relies on fighting head on with raw power mostly, cavalry relies on good manuever in the battlefield.
Don't get me wrong, some elite tier players prefer STR over AGI. But everything that can be done with high STR can also be done with high AGI, even if it takes a longer time sometimes. But the opposite is not exclusively true. People who wanted to have some athletics preferred to get more STR mostly (less points needed for both riding and athletics, easier to maximize the usefulness of attributes).
Also keep this in mind: Bump is one of the greatest weapons in the arsenal of a cavalry. STR doesn't improve it, but AGI does.
While theoretically this all is 100% true, the last part is practically impossible for some peole without destroying their whole character/build by retiring or respeccing a level 33, 34 or even 35 char they don't want to retire or respec.
Thats the whole point here for some people.
Exactly my point: "It will only take them time/gold to achieve it, nothing more."