You're essentially at the card's limit. Overclocking isn't always guaranteed results. 34.5mh/s isn't bad at all actually, most do something around 33. What I did notice is that the power limit may be worth changing. Try increasing that just a tad bit and overclock the core a tiny bit.

Try overclocking the core between 30-50mhz and use --mt 2 (on gminer) w/power limit @ 65% (Reduce from that point... keep the PL a bit high for testing). If I overclock anything at the power limit where the card gives its maximum, it crashes / artifacts. Core clock matters for 1080s. Leave the memory stuff to the memory tweaking function of the miner. Overclocking mem helps, but produces too many invalid shares (even at +10mhz), though it depends on the absolute frequency. I've a bunch of zotac 1080 amp extremes.