Many folks have the '5 GHz or it's useless' mindset...despite the fact that the only thing that beats a stock 9900K in gaming is an overclocked 9900K...but most of those comparisons are done with 2080Ti cards that will actually allow/show a difference, and, mostly at 1080P resolution.
Even at stock (non-MCE enabled) settings, it will turbo to 4.7 GHz on all cores, which is hardly a 'I sure need more CPU!' scenario for most folks... (additionally, most folks don't have enough GPU and/or a high refresh rate monitor to effectively utilize the extra potential FPS anyway)
Additionally, although MCE will allow the BIOS to direct higher than stock all core loadings of the processor, I'm not sure merely selecting MCE-enabled also raises the core voltage enough for 'guaranteed stability'. (I suspect a corresponding core voltage bump is often requried for many 9900K models...Although many (some would say most) 9900K's make 5 GHz on all cores, it is certainly not 'guaranteed'. You can, however, specify in the BIOS or from within INtel's XTU which clock speeds are maximum at assorted core loading scenarios, perhaps reducing the all-core speeds from the default MCE-enabled goal of 5 GHz to 4.8 GHz, testing temps, trying 4.9 GHz, and seeing how your cooling solution responds. A 9700K with an NH-D15 hit ~68C at all core 4.6 GHz loadings of CPU-z/stress testing, and, as wattage starts to increase rapidly near the top clock speeds and core voltages, it would be hard to fathom the NH-D15 being enough for your goals, short of playing outdoors in 8C ambient temps.
Most generally accept that all-core 5 GHz action requires a healthy quality 280 mm/360 mm radiator fluid solution.....or luck...or both. (The KF-series of CPUs may run a few C cooler as well)