9900k should allow two cores to boost to 5.0 GHz as normal behavior if I recall correctly...(this can be verified by observing HWMonitor under load...)
a few concerns about overclocking while gaming-- often, unless running a GPU powerful enough where this overclocked CPU load can be actually 'harvested', often running all cores at 140-150 watts/5.0 GHz (done automatically in MCE mode) is often just a waste of power and added CPU heat...
This would likely mean needing a 2070 or higher GPU, at 1080/1440P
@144 Hz monitor, before most would even worry about needing all cores at 5.1 Ghz, etc... (certainly it would be a waste of time with the likes of a GTX1060, etc., which is maxed out even with a stock 6700K at 4.2 GHz...; almost doubling power and heat to boost from 135 fps to 136 fps seems pointless to me, anyway..)