It's true. With 12.5 Claymore I have on my 280X 330-335 Sol/s with modded bios, overclocked memory and downvolted core.
Optiminer gives only 275-300 Sol/s.

The problem possibly was in intensity 8. In 280X is 3Gb video memory - it enough only for intensity 7 in Claymore 12.5. If miner runs on intensity 8 the speed will be slower because of out of memory. In my system miner don't run in intensuty 8 and 9 saing that not enough video memory. But in other system/driver it can run at intensity 8, but speed wil be much slower...

You have 330 with stock bios and overclock. I have 335 with modded bios, light overclock and downvolting. Power draw of my card lower. Speed the same.

I have high temperature of the card because it's not only one in my system block ))))

1. System - Windows 10 Pro x64 CU, licensed with all latest updates.
2. Driver - AMD Crimson ReLive 17.4.1 - it's only one driver with wattman for GCN1 cards. But I have the same speed on all new drivers up to 17.7.2.
3. Card - ASUS R9280X-DC2T-3GD5. Hynix memory.
4. Stock freq. - 1070/1600. Speed not measured because I long time use modified bios with non standard memory timings.
5. Stock vcore - 1.2 V.
6. I use 1085/1665 freq. and 1.1 V core voltage.
7. Claymore's ZCash AMD GPU Miner v12.5. Intensity is 7. Monitor not connected to card. If monitor will be connected than Intensuty will be 6 to have enough vmem to video output to monitor. Speed difference between 6 and 7 intesity is about 5 Sol/s.
Command line:
ZecMiner64.exe -zpool(...) -zwal(...) -zpsw x -mport 0 -allpools 1 -i 7 -asm 1

I noted that Claymore ZCash miner is very sensitive to temperature of the card. If temperature is high miner became unstable. Even if temperature not critical. F.e. on my card on other miners (and Calymore other algo's miners too) my 280X stable mines up to 90-92 Celcius. But Claymore ZCash miner on temperature higher than 86 start to stop video driver...