I play a lot of CS:GO (maybe not so much lately, but I have over 1700 hours total). The most important thing for me was having a monitor that does not put me at a disadvantage compared to other players, but is still able to produce a decent image. Meaning excellent performance for competitive games, but
at least good enough performance for other tasks.
If you're in the same boat, then what I'm saying is quite simple: shaving every last bit of input lag is not really important. A difference of 3 or 4ms really doesn't matter and is not worth sacrificing image quality, unless you're in a LAN setting where network latency doesn't exist (it's insignificant compared to server tickrate latency.) And even there, the situations where a 4ms difference actually influences an outcome are few and far between. How many times are two actions performed by two players closer than 5ms apart? It doesn't seem plausible that this happens more than maybe once every blue moon.
Where I'm going with this is that virtually all gaming monitors will give you excellent competitive gaming performance. So I would go the reverse route of what you're trying to do: compile a list of high-refresh (144Hz and up) gaming monitors that have the best image quality characteristics, and don't even consider the ones that have obvious image quality issues. Then see which of those that are left have the lowest input lag, highest refresh rates and best image quality.
As for G-Sync and ULMB, those are usually not used in competitive gaming, since tearing is not important there, and motion blur is "good enough" at 144Hz and up for most competitive games. They are still awesome to have for non-competitive gaming though.
(There's still a use for ULMB in some games where motion clarity is more important than input lag. Chief's example here has always been scanning the ground while flying a helicopter in, say, Battlefield. Although I'm not sure Battlefield and other games like it count as competitive.)
So is the PG258Q a bad monitor? Nope. It actually does tick the "excellent competitive gaming performance" checkbox. However, at $600 it gets you awfully close to a 1440p 165Hz 27" G-Sync IPS screen (about $30 difference) like the
AG271QG, which are still very good for competitive gaming, but offer a quite improved image quality.
If the PG258Q would cost $500 or less, then I'd say sure, solid choice. But right now, I'm not sure its input lag difference is big enough to compromise image quality
at that price.
(As a side-note: at least for CS:GO, player model size actually helps, and since on 27" monitors things are bigger, you have bigger heads to shoot at
)