I have seen them stacked before. PT&T against the transom and jack plate bolted to it, then the motor. Gives it 11" of setback, and it was a tiller steer motor. Was it fast? I don't know. I didn't stick around to watch the boat races. I would imagine trying to steer that thing through the flooded timber with the 11" of setback would be interesting to say the least.
Performance wise, if your current manual tilt motor is currently bolted to the transom, you're far better off with PT&T than with only a jack plate. A jack plate won't let you tilt the motor up with the flip of a switch, so you'll be still manually pulling on the top of the cowling to get it tilted.
Bob's machine has a PT&T/Jack plate combo for smaller motors. it's awesome. It's pretty. And it's expensive. But they flat work. On one boat that I know of, it picked him up from 31.3 mph (bolted to the transom, av plate level with the hull) to 37.8 with the bob's combo. But for almost $1800, it better pick it up! If I had the money to spare, I'd have one. For what I use my boat for, it'd be handy. Mine's a big ol' heavy 4 stroke and manual tilt, so PT&T is almost required and I use the boat often in a shallow river where we find ourselves dragging bottom fairly often.
Usually (not always) a properly set up PT&T should pick up the speed about 2-3 mph. And you get the benefit of more tuck (on plane faster with less bow rise) and obviously power tilt for when you're trailering or jumping over a log or whatever.