Heirlooming isnt really worth it AT ALL.
Even in full +3 you'll be only around 5% more resilient/stronger/faster.
This +5% can save your ass some time, but 95% of your skill is keyboard/mouse + build; heirlooming is for me only a long term goal and a desire to be the best at all cost. Its an efficient way to enhance the replayability of the game
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BUT, the most "interesting items" to heirlooms are definitely those that :
- puts you in a "very high" item value (like heirlooming heaviest armors, or faster/stronger weapon; it then becomes "unbeatable" in its category)
- have multi-layered heirloom item bonus : armors have only 1-2 bonus values (1 armor type, or 2 (for the chest, or even 3 in the case of the gothic plate with bevor)); the horses have 4-5 stats bonus (armor, hit points, speed, manoeuvrability AND CHARGE) and a few weapons have 3 stats bonus (speed, damage and weight)
Those are definitely the first to be heirloomed if you want to be the most efficient;
If you want style, heirloom what you want!