Okay, right, I see you're not very familiar with computers in regards to these things, so I will explain some basics.

DDR stands for Double Data Rate. This is why all RAM is listed as having, for example a 1600MHz clock speed but programs report it as 800MHz - because the modules actually work at double the effective frequency (technically it's because data is transferred at the start and the end of the clock but I won't confuse you with this stuff). This has nothing to do with how many sticks of memory you have...that's only related to Dual/Triple/Quadruple channel memory, which doubles etc. the speed at which the CPU communicates with the memory, not necessarily connected to what clock frequency the RAM runs at. On modern systems this is entirely dependent on what slots you put your modules in, they have to be in specific slots for this to work. And no, you don't have 2 sticks of RAM ... you only have 1, which leaves you with 3 empty ones.

Your benchmark results may be lower because of your settings. Make sure you're running High Performance and running newest drivers for everything. The difference between 8 and 24 GIGAbytes (they're not MEGAbytes, as you've written them) is very significant in heavy workloads such as video editing, image manipulation software etc. Not sure about benchmarks but yes, that does play SOME role, however small it may be.

I'm not going to recommend overlocking to you before you become better informed with technical information and the risks involved. These computers don't differ from each other in terms of performance much so whatever you have getting you lower results is probably entirely software based (other than the memory amount, that is). Check other G751 results that have 8GB configurations and compare with them instead, on other websites.
It may just be that the tripled system memory is what caused the difference, I don't know what tests that particular benchmark does.

If you want to buy a SSD, in the description of the product it should say what interface it uses. All (save for a couple newer models) 2.5 inch standard drives that you see everywhere are SATAIII based SSDs. There are M.2 stick-shaped SSD drives that can also be SATAIII, watch out for this. The G751 supports the PCI-E 3.0 x4 interface standard so something like a Samsung XP941 would work for you if you have the slot (check that you do, some models don't come with the slot, it's above the left drive bay, to the left of the RAM slots under the access cover on the bottom). You will also need to find some way to secure a M.2 PCI-E SSD into the bay as the G751 does not come with a mount for it if your model didn't originally come with such an SSD. Aeolisio, here on the forums, has designed such a thing that you can have 3D printed for you. For a SATAIII drive, something like a Samsung 850 EVO would do just fine.

Ooops, of course 8Gb or 24GB was meant, thanks !

Thanks for the RAM /DDR speed explanation, very good to know.

I have updated the graphics drivers already and it's on high performance, so the result just worried me a bit. I'll check regarding other drivers and will try some other benchmarks.

The Laptop has an SSD already. The question was related to the sticky thread above, where it was recommended to check new Laptops if they have SATA or PCIE. It's not mentioned on the box or spec sheet or whatever.
Of course I'd like to get one with PCIE, but it's still not quite clear how to check for it ?

It's probably the RAM then, 3 times the system memory is a considerable difference in any computer.

Right, I'm guessing you bought this in Germany, yes? I'm reading the specs on the websites and they all say 256GB SSD for this model.

I would assume you have a M.2 PCI-E 256GB XP941 Samsung drive in there.

Easiest way to check is go to This PC / My Computer / Computer etc. depends on your OS and right-click the SSD there and go to the Hardware tab. There you will see the product name.

Otherwise, what I woudl do is just open the back, it's user accessible, you just have to loosen one screw (don't try to remove it, just loosen it) and pop the back off. If there's two 2.5 inch regular type drives in there then it's SATAIII but if in the left bay there's a long, stick-shaped drive over a metal plate, like the next image, then it's M.2, which is the better of the two.

Devil is in the details. Looks like they are quoting the graphics score and not the combined score. They don't provide the benchmark score for 2013 either they call out the specific scores for some reason. Possibly because the numbers are generally higher...