You probably won't see performance gains from upgrading memory, unless you are using it all. No, it won't be any more stable due to being 1.5 V. Try running your existing memory at 1333 MHz and 1.5 V – you should get the same [marginally less actually, but not enough to make a difference] performance, and it should be stable.
No, it won't be a more stable setup because of the 1.5V rating but it will be more stable because it runs a 9-9-9-24 specs.
And yes, 1333Mhz@1.5V does make it stable, but 1600MHz @1.5V is also the same, performance difference is minimal as far as human speeds are concerned.
1600MHz@1.65 does make the system a lot less stabler because of the increase in standard (1.5 to 1.65) current also comes into the picture.