This is a wet dream for all the Dapps out there.
I couldn't think of any flaws in this design. This is perfectly designed and is a huge achievement for Hive and I'm really happy to see this happening. I will certainly give it more thought later.
We will need to use all the marketing we can get to claim this achievement in the crypto space.
This would really help Honeycomb, we run irreversible for consensus and head for API.
From my experience most dapps just use head and hope no reversals actually affect them. So for most dapps this won't be a performance altering change but will put them at less risk if they do decide to switch to irreversible instead of head.
It will be a big performance change for dapps using HAF, and I hope that before long most new dapps will be written using HAF (the performance benefits are substantial for any dapp that wants to scale).
But for existing dapps that just directly process the head block, you're correct: it won't speed them up, it'll make them more resistant to failures in the face of forks (arguably more important than making them faster, of course).
It also wouldn't surprise me if some existing apps wait a block or two (IIRC, hivemind does, for example).