Have you ever been a HA?
The accuracy is shit (even with 15/24, most people slow down hugely to shoot), plus you need a lot of riding to dodge enemy arrows and cav.
So this build would probably suck. You would be more like a foot archer with higher mobility but with nerfed damage from being on horseback, and you would need to be nearly stopped to shoot even close to accurately.
That said, I have not tried that build, but I used a 15/24 on an alt for a couple of gens in an alt (and sucked at it though on foot i was not too bad) and the reticle was quite big already (with 4 HA, only 5 PD, much more wpf especially those +28 from the PD difference).