Matriarch is definitely the hardest, but while Hans haven't scared me in years... I still struggle against the Patriarch unless my team is very coordinated. Admittedly, I learned how to deal with him fairly well, even in solo, but he can still chew through players quite fast. But there's no denying that the King Fleshpound is laughable. Sure, his laser beam can deal plenty of damage... but the fact you can simply crouch and dodge it makes the boss a real breeze to defeat.
As for OP's idea though... Yeaaah, no. I must say that I also believe that turning regular zeds into bosses has already grown old. Unless you give them plenty of new abilities, they'll always fall short compared to bosses thought from the beginning as bosses. And even with new abilities, well, that's my point... why not come up with a new boss from the get-go?
Instant kill also doesn't sound fun at all. I'm all for challenging bosses, but that's a bit much, especially if you're a solo player. And since the rest of his kit is pretty much the base Scrake, but stronger... It's really not a very captivating idea at all. Bullet sponges are rarely favored by players.