We would need to know which heirloom variety you are looking at, but look at it this way, barley breeders have been optimizing certain traits for malting for over a hundred years. Klages, which was developed in 1974, is almost non-existent today. Gives you some idea of how fast the improvements come.
I've planted heirloom tomatoes and never got a single tomato off of any of them!
there is also something to be said for genetic diversity. heirloom varieties are extremely important if a disease or insect pest effects a crop as they contain genetic diversity that has been breed out to exemplify a more desired trait. they may be what allows us to brew beer in another 50 years when changing climate makes our current varieties less fruitful......just something to chew on while...