I am not sure what the Heirloom Professional Grade rear would look like on its own. I have a Signature done by them and they flatten top of the slide and the back of the sight is cut to fit the slide. All other HP sights I have seen installed they cut the slide for the sight at the back and you end up with a shelf looking area to each side of the sight.
The rear sights are quite similar. The PG sight has more of a "Ledge" at the front of the slide which assists in one handed manipulation. (racking the slide off your belt, pants, etc. Heinie also offers a rear sight with a "Ledge" at the front. Again, both are similar but I prefer to support the Heirloom gang because of the charity work they do with Vet's and LE. :rock:
Just a thought... what about Novaks? They seem to be "made" for the Hi Power. Not large, good sight picture, no sharp edges, etc. Stephen Camp has lots of photos of his Hi Powers with Novaks. I do like the Heinies, though. Nice guy and an awesome pistolsmith.