My thought is that if the OTL 4.7"/45 BL MKI is just in service and that simply using I would save money and should be acceptable ?
So a 4.7"/45BL in a twin turret to give better control and rate of fire (with more protection for crew and higher elevation)
That or design the QF version (MK IX) (with split shell/change) earlier (should be cheaper than the all new MK VIII) and then it would replace the MK I on most 20s built DDs (maybe some with HA mounts later
)
IMO (maybe with a bit of hindsight) 10x4.7" (on each side) should be quite acceptable v 6x6" OTL, I think the extra RoF (more guns, each faster to reload using a smaller shell) would cancel out the bigger shell effect ?
And yes with hindsight I am solving the RNs mid cal weapon 'problems' in 1923 for the 1920s-45+ (maybe till 60s
and then buy in a 5"OTO/USN gun)