Properly tuned, I don't see how you'd see any real difference between a
SU and a mikuni. I can see tiny improvements from using fuel injection,
just because you may see better atomization....

I wouldn't say tiny, small would be a better description. The beauty of SU's is that they do provide optimum fuel atomization. When fitted to the rather poor manifold that the standard 2x SU manifold is SU's will provide more torque & full throttle fuel economy all the way up to when they start to become restrictive.

If I had a dyno and a couple weeks of time, I'm sure I could come up with
a custom needle taper that would work perfectly with my wacky B20.

Don't forget the spring, it's of equal importance to the needle. The spring must not let the piston run out of travel. Ideally the piston should approach full lift at or just before peak horsepower.

I would actually like to hear about the holley, especially if it could be
adapted to the (somewhat) common weber downdraft manifold.

It's been done to death on 2.0l OHC Ford Pinto's. Pinto's come with the Weber DGAV & a reasonable manifold(just like the IPD kit). 350 & 500 cfm Holleys are a cheap upgrade after the DGAV runs out of CFM. To make them run on 4 cylinder you need to reduce the fuel ports size behind the power valve. I did this to a friends Ford Escort & we were extremely impressed with how tractable & economical the Holley was.

I'd also like think about using the lynx manifold with a pair of carbs like
the mikunis. I'd imagine that the inner vs outer throats would require
slightly different tunings given the length of the runners in that setup.

The Lynx maifold is interesting. I had one for awhile & I kick myself for not trying 2x SU's on it. The different length of the runners is far less important than the sharing that goes on with the standard 2x SU manifold.

Lastly, let me add that I think there is a market for a totally self-contained fuel injection (throttle body style) setup in california to replace
carburetors on cars where people don't want to junk their older cars but
where the state wants the owner to clean up the car's emissions.

It's not really the carburettor that controls emissions. It's a function of camshaft overlap, compression & ignition timing.