Jamie: 4AGE has injectors in the head, not on the manifold.
Doug: Apart from being harder to fit a 20v in (mostly due to the dizzy, and also due to the water piping), the torque curves on the 20v are also a fair bit different, as far as I've heard. The 20v's are a bit like the Honda VTECs, they make good power, but they make it all in a fairly narrow powerband. I've been told that the 16v is a far more flexible engine.
That said, it's all stuff I've heard, nothing I know as a fact.
Anyways, to drop the 20v in, you'd have to convert to either DLI or do a dodgy and move the dizzy up front (like Acoustic had on his 20v for a little while). Or mount the engine further forward so that the dizzy clears.
And after all of that work, you'll have a stock 20v.
Aftermarket computer on the engine would give a hell of a lot more options, not only would you be able to get your engine running *perfectly* for it's current setup (quads or no quads), but you can run bigger cams etc etc etc, things you can't do with the standard computer.
Putting big cams in a 20v is hard, because of the VVTi. Putting adjustable cam gears is also hard, again because of the VVTi. The 20v engine is a better engine in some ways to the 16v, but it makes it harder to make it go any faster...