Are you aware of the existence of the Malanthrope? It's the latest ForgeWorld offering for the Tyranids, and it's basically a giant Ripper which can cause some serious pain if used correctly. Sounds like a possible substitute for the Tyrant.
Otherwise I'm a big fan of the idea and would love to see some more. I think it's worth bearing in mind that you can make rippers fast attack by giving them wings, might help make your list a little more flexible.
I really like the idea of lots of rippers as basic troops to bog down larger units and then Carnifex etc working as support to take the bigger stuff down. I'd love to hear how it plays as well.
Godspeed sir!