Imps are just so good
Maybe I wouldn't exactly say that it's strong generically. I'd say it's strong to open with because of Imps, pretty strong early, ok overall, and well-designed because getting imps is hard. With the modification, it's very strong at every point and pretty boring.
the powerlevel question seems noncentral though; the point is that it takes away interesting decisions.
Which interesting decision is taken away? Be concrete, what would you do with the Heirloom that you would not do otherwise (except getting Tormentor more often which is not about decisions but the power level of the card).
Tormentor is a very weak $5 which you will never ever buy early if there is a junker or trasher. You will only buy it later if you got the terminal space for a terminal Silver with a weak Attack on top, which is quite rare.
Of course gaining Imps is very strong (but then again, with Lab variants in the Kingdom, Haggler can do the very same thing far more consistently) but you cannot pull that off often with Tormentor so why not buff it? It is not like we talk about buffing Mountebank, Cultist, Coven or some other ridiculously overpowered Attack but buffing a very weak and random Attack.
There is a reason so many Hex Attacks violate the terminal Attack principle: the Attack is weak and random. Nobody would e.g. ever get Skulk because of the Attack, you either want the Gold or are in dire need of the extra Buy. The Hex Attack never matters decisions-wise.