There's a fairly lengthy post here regarding that laptop model, 16GB of RAM and performance in general.
On the whole, it seems to work as far as hardware support goes, but there are significant doubts about how noticeable it is from a users perspective.
Some of the key paragraphs of note are:
This is a further explanation of where I think this notebook benefits best from its 1333Mhz memory allocation, as we know it is capable of using up to 16GB SDRAM, in a matched or random batch of high-quality 1333Mhz SODIMMs (be they 4GB SODIMMs in 2x 8GB matched "kits" or simply 4GB SODIMMs bought individually and assembled into a 16GB allocation of RAM); we also know it can support 12GB RAM in the configuration that I extensively tested for our mutual benefits previously, which is 2 x 2GB 1333Mhz SODIMMs paired with 2 x 4GB 1333Mhz SODIMMs, either an 8GB "Kit" or a pair of 4GB individual SODIMMs, totaling 12GB RAM, and I found that this particular combo of 12GB yielded the highest througput in Everest Utltimate Benchmark Testing, across the board, plus it tested to be much more stable than 16GB did overall, as the memory controller gets "overloaded" or shall we say "saturated" at the 16GB quotient, and the notebook suffered freezes, and significant performance bottlenecks with 16GB vs 12GB RAM, or so I found testing the Corsair brand of RAM in this notebook.
So that's the story tonight, and I am here to report it faithfully to all you who are thinking of using more than 8GB of RAM in the computer, and basically guys and gals, if any, it seems to be a waste of good old All-American $$$'s to stock up one of the G51's with 12GB or 16GB of RAM...it might turn a few heads, and it might be of some benefit when one is using multiple high-yield RAM programs at the same time while working hard on a project or assignment of whatever sort you can think of...but overall using 6GB or 8GB of RAM in this notebook seems to be the logical way to go with it! That idea is congealed from much thinking about this subject, a ton of testing results and efforts both in my notebook and SassiMac's to boot, albeit his being virtually new and mine being an earlier example of the breed, they are both G51JX-series notebooks through and through, and representative of the genre such as it is, and I feel 100% justified in writing the conclusions I came to in the past couple of days, and strange though they are, they are what they are, end of story!