Obviously we want to be able to fill a rack, and in order to do so, we need to provide enough power to each rack.
Which is the hardest part of designing and running a datacenter.
Are there cases where more than 6000W per rack would be needed?
Yes. We are seeing 10kw rack, and requests for 15 to 20kw are starting to come in. Think blade.
(We are not worried about cooling due to the special circumstances of the space.)
You've already lost.
Would someone pay extra for > 7KW in a rack? What would be the maximum you could ever see yourself needing in order to power all 42U ?
These days, there is not (or should not) be a connection between rack pricing and what you charge for power. As for how much, ask HP or IBM or whoever how many blades they can shove in 42U.