I dont have much experience with all heartwood yew bows. I have a lightweight target bow just about finished up that is all heartwood backed with salmon skins. Not a very stressed design, but it it shoots nice, no sign of problems.
I have seen several old yew bows that were all heartwood and they looked pretty stout too. The Kalapuya made some all heartwood bows that were short, wide-limbed recurves.
I imagine you can get away with a bit more with the tight-grained stuff than the course stuff. Lets here more about this wood. Pics?