As I look at the G751's underside, I don't see much holes for air flow. Perhaps you need a cooler that forces in air in that one spot.
The G752 however has a removable back (Which I assume isn't supposed to be removed, but I do it anyway).
https://www.youtube.com/watch?v=rwSuiEEjU-E <-- Funny there's actually someone here who did it on YouTube, that makes it easy to explain, woohoo.
Doing this plus using Zalman's most highly coveted NC series cooler reduces my temperatures 8-10*C from 84 to 74-76.
The NC Series uses an aluminium plate, which is always cold to begin with, it also has a big surface area, so not only does it provide air to the whole intake, but it also catches and traps all the dust and crap it sucks up, keeping it out of my gorgeous lappy. ;x
Because your intake's smack bang right in the middle, it looks like the pad you bought is the best for the job.
Best way to tell if it's doing a difference is grab this,
https://openhardwaremonitor.org/Download, install it, run it, play a game for 30 mins, or however long.. note your highest temperature. Turn off the game, reset the temp records, then fire up the game again for another 30 mins. (Don't move around, just stay in the same place to keep your test consistent.)
I think with a model like yours, you'll see maybe 5*C drop max.
❤