To illustrate eHalcyon's point, if Heirlooms just substituted one Copper you the possible opening moves would be 2-6, 3-5, 4-4. This is still too opening-changing for my taste but Heirlooms substituting two Coppers is even more radicall:
You could e.g. start with 6$, buy Gold and Heirlooms is trashed; draw 3 Estates, Copper and Gold on your second turn and thus have 4$. The probability to open 6-4 is pretty small (1/42: chance to open with 6$ is only 5/42 and chance to draw the Gold on turn 2 is 1/5) but it illustrates the increased opening volatility.
It is most likely to open with 4-4.
If you open with 5 Coppers (1/21) you could draw (1/6) and play that 5$ card as well as Heirlooms on turn 2 .
If you open with Heirloom, three Coppers and an Estate which is not that unlikely you have to trash the Heirlooms but have a chance to play the 5$ card on turn 2. If you did not draw it chances are high that you draw a Copper so you can at least buy a Silver.
I find it hard to imagine a situation in which you would not wanna trash Heirlooms for a 5$ Action card.
