The max QPI from Intel is 1.35V. See table 2-6 in their datasheet. http://download.intel.com/design/processor/datashts/320834.pdf
However you likely won't have to go that high. I've seen recommendations for keeping DRAM and QPI no more than 0.5V apart, which is the opposite of what you stated but may be what you are thinking of. In general, some folks have had trouble getting their RAM to run stable once bumped up to 1.65V without also raising their QPI / Vtt to 1.15 and thus keeping a difference of no more than 0.5V between the two.