HTAs don't deal more damage. Jarids deal 30% more damage, but they do deal less damage than HTAs under 22 armour. 15-24 also only deals 2-3 points less than 21-18, it's just that it feels that the 5PT build's advantages outside of athletics feel similar enough to just take the extra PT. 15-24 is great for HT though.
Better performance? Perhaps, but that would definitely surprise me.
Are you certain you aren't receiving a damage penalty from hitting limbs? I forget the specific numbers, but two 74 raw damage shots against a lightly armoured opponent resulted in something like... 20 damage on the first hit and 40 on the second (I'm fuzzy on the exact numbers, but I remember dealing twice as much final damage).