Horsepower Q?

There's really no such thing as a stop point. You will virtually ALWAYS make more power with a correctly sized set of long tubes.

It's not as much a matter of ability to flow as it is the ability of the exhaust system to extract (scavenge) spent/lazy gasses from the exhaust ports of your heads (and cylinders beyond that).

There's a bit more to exhaust than just that but that is the BIG advantage of long tube headers. Those gasses gain velocity down the tubes and the venturi creates a vacuum that pulls other spent gasses with it.

Like anything, long tubes can be incorrectly sized for an engine combo just like shorties. The trick is to find a complete exhaust system that appropriately sized for your application.