Thanks for the response.
Added "diff-factor" : "2" to the config file and gave it a try.
8.11-Klaus-T quickly found 1 share which was rejected with a reason of "reject reason: low difficulty share of 0.23825932042784703" after which it apparently died. That is, it did nothing for the next 12 minutes, with task manager showing 0 CPU activity for the ccminer task.
The 2.1-tribus version ran with "diff-factor" : "2" in the config, and the effective hashrate in the pool was close to the 151.x hashrate shown by the miner. The problem with 2.1-tribus and the suggested option is the almost 50% rejection rate. After 1h 24m, tribus found 839 shares of which only 437 were accepted, the other 402 shares were rejected with the same "low difficulty share" message. During the same time, the amd cards had 548 accepted and 5 rejected shares.
The suggested option fixes the problem of the pool showing a very low effective hashrate for the nvidia worker, and increases the shares ccminer solves to a level that actually exceeds the % difference in hashrates reported by the two miners, but only at the cost of an unacceptably high rate of rejects.
I'll try a different pool tomorrow, and maybe the 2.0 x86 and x64 versions of ccminer if the pool switch doesn't help. Just out of curiosity, which version of ccminer would you recommend for mining myriad-groestl? I like the fact that 8.11-Klaus-T shows a 10Mh/s higher hashrate than the other miners, but that is only useful if the higher hashrate translates into more shares.
I'll post what I find tomorrow after the pool switch.
If you have half the hashrate on a pool, or too many low difficulty share rejects then it's usually caused by the pool and the miner not being on the same page regarding what they consider the same amount of work (difficulty) due ton how they were configured. With -F 2 the miner also submits work to the pool that is half of what the pool tells the miner is required.
If you have too many rejects with -F 2, then you should lower it (1.5 or something) until you figure out the correct number which hopefully gets you your hashrate reported on the pool without rejects.