I suspect they also realised that having 200 UKSK launch tubes on big ships sounds fantastic but would actually be rather expensive to fill.
Having a bigger launch tube means bigger missiles can be developed and used... the INF treaty did not apply to ship launched missiles... the American Navy insisted on that, and the US Air Force also insisted that air launched missiles are not included either.
Having a bigger tube could mean the UKSK-M could use S-500 long range SAMs/ABM missiles, but would also mean that if they want to fit SAMs in it then it needs to be able to stack, and of course a larger tube is always better for stacking than a smaller tube.
Onyx is in production and use today, so any upgrades can immediately improve performance across the fleet... interesting that they probably limited its speed because their is a land based shore battery version of the Onyx... the downgraded Yakhont is for export and has a range of about 300km, but the Russian Onyx probably can reach further... perhaps 400-450km while complying with the INF treaty.
Whether ship or land launched range performance will be similar so an 800km range shore launched anti ship missile... with lighter electronics, probably lighter stronger materials in its design, improved fuel, and likely improved motor design, though probably still a ramjet rather than a scramjet, should significantly improve performance.
How's about that Onyx with Mach 4.5 speeds?
The increased power fuel should increase thrust so while the new Onyx might only have its engine running half as long, the extra thrust might give it a higher speed and therefore longer range.
Say for instance it weighs 3 tons at launch, with 750kgs being payload and electronics and surface controls etc, and perhaps 750kgs of solid rocket fuel to lift the missile into the air and get it accelerating and climbing, that would be 1.5 tons left for fuel, so moving at mach 2.5 for say 350km, that is an 800m/s moving object flying 350km which should take 437 seconds, or about 7.2 minutes... the solid rocket motor would burn out very rapidly and speed should increase as it travels because it will be getting lighter as it flys.
A new fuel, with perhaps both more energy and more volume might take up 1.8 tons of the weight of the original missile, but at mach 4.5, which means it is moving at about 1,440m/s, so 800km would take 555 seconds with 9.2 minutes of burn time with higher thrust.