So I have 2 separate characters at 50 with Afterburner and Fly now, and I can post some results I've seen. Both have 2 50+5 Fly IOs in Afterburner, which puts the flight speed cap at 86.68 mph. One has a 50+3* Fly IO in Fly, that and 2 Moderate Improved Movement set bonuses (the 6% one) puts her at the 86.68 mph cap I mentioned. The other has a 50+5 Fly IO in Fly, and that puts him at 86.23 mph flight speed. Swift is not slotted for flight speed on either. So I'd say that two 50+5 Fly IOs in Afterburner and a 50+5 Fly IO in Fly is a pretty efficient and effective slotting.
*The 50+3 isn't some special number I calculated to hit the flight speed cap, it's just how many enhancement boosters I'd put on that IO before I thought to check if I was already flight speed-capped, and I was.