I own a different ASUS G53SX, and I run 16 GB of RAM in it. It is a lot of work to upgrade, but
(A) yes it will support 16 GB of RAM. The speed of the RAM is your call, but it should be at least 1333 MHz. The BIOS will spit out a message that system specifications have changed, and ask you to enter the BIOS to verify. All that should change is the list of the amount of RAM in MB. It should be 16384 MB for 16 GB of RAM.
(B) I have not noticed that much difference, except when I run VMs. It is very handy to have the 16 GB of RAM, so that a VM can have at least 4 GB to play in, but the host system still has plenty of memory. I originally had 8 GB of RAM in mine, and it was worth it to upgrade. For gaming, which I don't do much of, it may be iffy, but if you are going to go to all the trouble of opening the system--download the instruction manual! You might as well pack it to the gills.
While the system is apart, make sure (if you have the budget) to add a
Seagate Momentus XT drive. The hybrid drive cuts boot times in half when compared with a 5400 RPM Hard Disk Drive, which is what most of the cheaper G53SX machines come with.
(C) ASUS manual says that the modification is not covered, but there is no danger to machine if you are careful. Just watch yourself when you take the machine apart, as many of the screws are tiny. If something happens to your machine, simply put it back to the factory setup--remove all user-modifications--and send it back for repairs.
In short, go ahead and upgrade. If your machine has only 4 GB, buy two sets of 2x4 GB 1333/1600 MHz RAM. I run 1333 MHz RAM, which is plenty fast for my needs, and it is what the system originally had. Assuming money is no object, go ahead and buy the 1600 MHz RAM, as it may run at the faster speed, which will improve your game performance. If it has 8, buy one 2x4 GB. The G53SX is a quad-slot config, which makes the 16 GB of RAM upgrade dirt cheap.