Well you can shape any shell to anything other then a factory set of specks and it's a new "wild-cat". But rather then asking myself 'if I can', I usually ask "why should I?"

5%? (even if that were the case, which I can't see in my calculations)

How about a 280 Remington instead?
Or a 7X64?
Or for that matter, a 7X61 S&H
Or a 7MM Rem Mag?

All these cartridges will do better then 5% increases in capacity, and you'd not have to order a custom set of dies, and a custom chamber reamer, both of which will cost a lot more then factory spec dies or reamer---- and IF you could get a bit more powder in your wildcat than you could in the standard 7X57,, how could it be any better (or as good) as any of the shells I just named?

Yes you can do this, but why would you want to?