I have a
G15 Refresh and a
G402 Hyperion Fury. Does it make a difference?
For the keyboard, that's a clear no. So why did I get it? Easy: I'm working 4-10 hours a day at my PC (actual work, not gaming) and I love the programmable makro keys! When I'm in my browser, I have stuff like ctrl+TAB, ctrl+shift+TAB, ctrl+F4 and ctrl+shift+t on those keys. ctrl+shift+TAB alone is worth the 20 or 30 Euro I paid extra (I can't remember anymore, because I got this keyboard more than five years ago already). When I switch to another program, the keys automatically switch the profile and I get shortcuts like ctrl+f, ctrl+n,ctrl+o or ctrl+s.
I don't use the makro keys for gaming. In 90% of keyboard+mouse games you have your fingers on WASD, shift and space and every other function is on adjacent keys. No need for makro keys. I can't say much about the "feel" of the keys. They go down when I press them and they come back when I release them. The keys on my old keyboard felt a bit more loose/shaky. But that keyboard was a lot older than my current one, so let's talk about this in five more years ;)
The mouse on the other hand, does make a difference in games. Not because of it's precise and fast sensor (I can't notice any difference at all), but, again, because of the makro keys. In a shooter, you can finally land a melee attack (f) or throw a grenade (g) while strafing right (d). Just put f and g on the two buttons next to your left mouse button ;) Awesome! And sniping is great too. You can put a lower DPI setting (mouse moves a lot slower) on one of the extra buttons, so aiming when zoomed in is a lot smoother and more precise.
Another ... "funny" example where it makes a very signifant "difference", would be Diablo 3. It's a very, VERY grindy game and there are basically only 5 keys on the keyboard that you need to be able to hit fast: the 4 hotkeys for your skills and the hotkey for the potion (you don't really need the potion, bt when you need it, you need it fast). So, if you have 5 programmable buttons on your mouse... you don't need the keyboard anymore! One hand on the mouse, the other hand in a bag of chips or at a bottle of beer :D That's perfect for games like Diablo 3! I wouldn't want to play it two-handed ever again.
The G402 is a great mouse. And it's not even THAT expensive. I got it for 35 Euro on Amazon (it's on sale all the time - currently for 36.99 on amazon.de) which is quite affordable.