I can't reach Claymore's speed with GG on 280X. Just try Intensity 7. Optiminer on 280X gives about 280-300 Sol/s. GG is max 250.
AMD Crimson ReLive 17.4.1 only one driver that give wattman for GCN1 cards and allow to change core voltage for each program by profiles. But it didn't allow to change memory frequences such as overdrive. I write my memory frequence through vbios and use wattman for over/down-volting.
Use AMD 15.12 drivers - best choise for old GCN1 cards. And you can overclock with MSI AB.
I got 325 sol/s with cm, i=7, core 1070, mem 1575, mod bios timings. I think, if I overclock it to "1085/1665 freq", I'll get the same results as you - 330-335 sol/s.