We here in Japan are routinely on 9-10km climbs averaging 10-12% grades with no less than 39x27. I've only got 12x25 10sp and just not having the 27 to shift in to will routinely let me outpace my ride buddy that has the 27 in the back. Does it hurt more? Probably, but we both spin 80ish, and I get to the top first almost always.
I think the question asked best is dialing in fitness to do the climb. If you're concerned about time, then getting a lower gear won't make you go faster. It'll just let you go slower for longer time. *shrug*
__________________
Envision, Energize, Enable