First, you should take into account that, even with a 64 bit OS, as long as you are running 32 bit apps (FSX and all its addons are 32 bit apps), you can't use more than 4GB per application.
Which means, an FSX fully loaded with addons will be forever binded into those 4GB, regardless of how much ram you put in. Addons that run as external .EXE files (like our own Couatl, for example) don't count into this limit, because each one can allocate its own address space, up to 4GB each, but many of the most demanding addons are in-process .DLL or .GAU, so they share the 4GB-maximum that FSX can use and, even if they are external .EXE files, when they allocated objects or do anything else using Simconnect, the objects they create and the things they do, happens inside the FSX memory space.
This means, expanding 16GB to use a 32 application, it's a waste. Of course, Windows can use the remaining memory for caching, speeding performances somewhat, but at that point you would be better served with an SSD drive than with additional memory.
Other than that, I don't see why you would have problems with 16GB, other than problems with your BIOS that has settings on RAM used or perhaps bugs with large quantities of memory OR you are using Windows 7 Home Basic, which can't allocate more than 8GB by design.
