Hi Chris, I don't think you need a 6" hole, a 4" should be plenty, remember the outlet from the direct vent motor is only 4".
I'd wait until you see the unit, and can measure, calculate, re-measure etc.
A 6" hole would give you some extra 'wiggle-room' and of course you can pack fiberglass insulation in the gap around the actual exhaust pipe.
Concrete cuts pretty easy. Now, a field stone foundation wall, that's another matter. Hitting a black granite rock in the middle of a 16" wall, that is a challenge. I was using a small jack-hammer, and it took a lot of time to get through.
When I've cut or drilled regular concrete, it is relatively soft, and cuts/drills easily