I am going to have to disagree with you on the HA builds Marathon, i think you missed the purpouse of HA, it is not about riding, stopping your horse, then shooting, nothing like that.
While you say there is no noticable difference between 2 and 4 HA i can assure you there is, beeing a dedicated HA for a while, every thing bellow 3 HA for a HA is useless, at 2 HA you can only shoot target which are 1 meters from you at full speed.
I dont find the damage to be such a problem with the 15/24 build. Of course, you are right on 1 point, with the 15/24 build you will have problems with tincans, but they only form a small portion of the enemy, HA purpous are not to kill the heavy inf, but to wipe out the light cavalry and skirmishers( archers, throwers, light inf and so on...) which you will do perfectly with the 15/24 build.
Your builds (27/12) cant really be described as HA, with this build you are forced to stop your horse each time to get an accurate shot, nothing horse archery is about.
In addition, if u are forced to go on foot,the high WPP (173) let you make very accurate shots, and if you have heiloomed bows, enemy armors wont be a problem