Definitely see a significant speed boost with game mode on using pcsx2. My Kingdom Hearts 2 Final Mix went from 103% speed idle to over 110% speed with game mode on! I don't think it'll make unplayable games playable, but will make your games run better and allow you to at a higher resolution.
The downside is that it'll also make anything besides emulation of pcsx2 wonky (meaning you cant really change settings while it's running) probably bc it suspends certain processes to increase speed, and will have to restart your Win to fix that issue. Completely worth it though.