My opinion is as follows.
IF one is a serious track day rider on a 919 AND:
1 is of at least a decent pace in the faster window or medium group, or slow race group.
2 knows how to do set up and make subsequent adjustments, or will learn to, or will pay someone else to do it.
3 has the budget and willingness to buy the F4(i) forks as cores for a subsequent full build.
THEN a full build of a F4(i) front end can be justified and exploited, otherwise it's a waste of $$$.
My guess is that the above will be less than 3 % of the 919 riders fleet.
(the above is aside from someone simply wanting to do a build for just the sake of doing a build, but most in that camp will go with a USD fork conversion)
For the all the rest, IF someone wants to vastly improve the front end within the confines of 04+ 919 forks, then a full build including a proper low speed compression bleed drilling of the compression valve body is the way to go.
IDK where I got this info, but someone was suggesting that USD forks really aren't worth it if you have to pay much more. In the case of the 919, you don't have an USD fork option without quite a bit more work. Meaning, the F4 series is as "direct bolt on" as you can ask for.
So the real question is "what do you get" for the F4 conversion.
The other concern is how you calculate the costs. If you remove the cost of the forks and just compare everything else, you have springs, valves and rebuild (seals, guides, fluid,...) then you can compare rebuilding stock 919 forks to rebuilding f4 forks.
THEN, the cost is JUST the forks. So that's $160~$200 ?
So you're looking at springs and valve conversion, say $400? (wild guess)
Seals, guides, fluid, say $100?
So the difference is what? $500 vs $700?
So what do you gain for your $200? That extra control valve at the bottom?
Compare that to the RC51 fork conversion for USD forks and you're looking at the whole front end and machining? Then you gain what?
It kinda looks like the F4 is a pretty good option IF your 919 forks are ready for a rebuild/valve upgrade. You spend about $200 more for something quite a bit better?