I'm not sure if I'm doing something wrong or what, but when I benchmark x16r on my 1080 I get 32MH/s with RavenMiner and 26MH/s on CCMiner - however when I actually start mining on ZergPool it gets 13MH/s on both.
Same problem with my mining cluster of 970's - 17MH/s when benchmarking, but 4.5MH/s when mining.
Could be related to difficulty used during the benchmark, and/or that x16r hops algorithms so your hashrate will depend on what algo it's currently chewing on. Best thing is to leave the high hashrate from the benchmark, sacrifice an hour of mining time and take an average hashrate from that hours worth of mining. Alternatively if you wanted to be conservative you could take the slowest hashrate during the hour.