From my experience there's a noticeable hashrate drop on Eth mining on Claymore with the driver anything other than the Blockchain one. And YES I tried the compute mode.

Blockchain hash: 28MH/s
Newer driver gaming mode: 20MH/s
Newer driver compute mode: 26MH/s

If you can't use more than 8 GPUs with the blockchain driver, I think it might be better to get a second system rather than having 12 GPUs on one system but getting lower hash/s.