OK, I'm waiting for both the risers and the molex connectors. I was glad  I could pay in BTC.

Meanwhile to pass the time I plugged one 8 pin cable from another psu to one of the underperforming cards  (the psu only had one) and the hash rate jumped by 20. So it is a power issue - but it's not exactly an underpowered psu - more like what you said - the mb not delivering the power.

I've read this post (#19) that made sense (not enough power for vc ram), plus  the main psu is probably not delivering enough either through those 8 pin connectors.

https://bitcointalk.org/index.php?topic=268879.msg2974007#msg2974007