increasing mem voltage will keep you memory stable when it's running in high frequency due to OCing - if you find you rams are running fine with stock voltage of 1.8V then there's no need to increase mem voltage.
post some cpu-z screenshot of CPU, Memory and SPD tab - I wanna see those.
looking at the temps of of you cpu I think you can push more ( around ~3-3.1 Ghz ) - just keep the cpu temp below 68C under load.
BTW, what app you are using for cpu load testing ?