9x140mm will be more efficient overall, as it covers more area of the Mo-Ra 420 and probably also will apply a higher static pressure. I did not run a 9x140 vs 4x200mm test by myself however I cannot imagine the Air / Water delta being anywhere near the 3 ° you mentioned. To give you some numbers of my setup:
Mo-Ra 420 4x200mm AF20 fans usually running at 450RPM while gaming (essentially silent, as my computer is right next to my desk)
I9 9900K
3080FTW3 at 105% PT (~ 400W)
Air / Water Delta in game between 6-8 ° C
I also checked the delta while having the fans run on full rpm (~ 800). This did not change too much, delta was improved by max 0.6 ° C.
I cannot image to achieve any improvements of more than 1-1.5 ° C by using the 9x140mm setup as opposed to the Noctua 200mm.
That being said, since 9xP14 is only half the price of a 4xNoctua setup and you apparently have no noise constraints due to the Mo-Ra being installed in another room I would probably save the money and go for the P14 setup. Cabling is annoying as you said but if the Mo-RA is hidden somewhere else anyway, who cares
* edit *
As [USER = 314775] @Midium [/ USER] suggested, a splitty is a good solution for your problem. However my understanding is that you are probably more hesitant because of the cable cluttering in the Mo-Ra itself?