Good news! Welcome to Nvidia mining.

I also had 280xs in the past which I also undervolted in order to save some precious energy. Sold all of them and bought the less power hungry GTX 750ti - 60 watt top, so really no need to undervolt them. To my knowledge people are not really undervolting these cards. Actually some are even increasing the TDP in order to be able to get all the juice out of this cards as it seems the default TDP is not actually at its 100%. I don't bother with that though, not worth it in my opinion and haven't honestly looked at it closely. We however OC those 750ti but the OC might vary on each algo. For example I am OCing my cards at +135 core /+ 200 mem. I am sure I could OC the Memory more but I just don't want to lose time doing that so I averaged the values and did a -5/10% to be safe. Depending on your brand/model you might be able to OC more or less. It's all about testing them.

As for having 6 GPUs on the same mobo, I have absolutely no problem with that and need no hacking of any kind in order for all of them to mine properly. I am running Windows 7 Ultimate 64x on a 990FXA-UD3