Q: Will the performance of FCP X improve if I upgrade the Ram from 8 GB to 16 GB.  Is it worth in upgrading and if so, by what % and in what work of it will it improve?

More RAM isn't faster; it just holds more data. If you don't run out, and you're not getting a lot of swapping, you're okay, and additional memory will not speed things up.

Use Activity Monitor to see if you need more memory. In Yosemite, check the "Memory Pressure" graph under the Memory tab. If it it's green, you're fine. If it's yellow, you might benefit from adding RAM, or maybe you're just running too many apps simultaneously. If it's red, buy more memory (assuming you don't have a huge number of apps open). This is in the help pages somewhere.

You can also sort the memory columns in Activity Monitor by size to see what the biggest memory-users are. I'm not sure which of the is useful for this purpose. (Click the column heading to sort by it. Click it again to reverse the sort.)

BTW, the worst app I encountered as far as memory usage goes on OS X is Firefox. I love its keyboard navigation and keyboard shortcuts, which beat that of all other browsers I've tried (Safari, Opera, IE, Chrome). But things don't work right in some situations, and its memory footprint cannot be reduced by closing tabs.

You might want to read https://support.apple.com/en-us/HT201464 , which explains more about Activity Monitor, including memory.