I wouldn't necessarily upgrade to 16GB DDR3... at some point you want to stop dumping money into an older system and at least the graphics card can be transferred to a new system.
Depends on the cost and how long you plan to keep it.
The bottleneck situation is hard to guess and as you said it "depends" though based on testing I'd say maybe 0% (no loss) to 20% loss in most modern titles for average FPS with periodic CPU bottlenecks causing stutter a bit more noticeable.
Even a GTX1080 will find many games where the CPU bottleneck is minor for average FPS at least... but do note other background programs can really spike usage when limited to only four older cores without hyperthreading which should cause more stutter (in some games)...