Smoothness

[Chief Blur Buster]

If with 240Hz máx VFreq monitor, user set 240Hz VRR-On VSync-On, with game at rock solid sustained 240FPS, and GPU using default Doble Buffer behavior, there are any lag difference Vs overclock monitor to 243Hz and RTSS framerate cap 240 ?

Don't bother.
The lag differential would be less than (1/237) - (1/240) = 0.053ms difference.
Unless there's a really good reason you need the 240 number for one reason or another.

Uncapped GSYNC at default settings creates input lag when frame rates max-out, as discovered in our 2014 analysis in www.blurbusters.com/gsync/preview2/ -- we were the world's first website to discover the "sudden-input-lag-increase" effect of framerates maxing out on a VRR display. Capping slightly below prevented this.