Again context…. Your average rider, rather than Bradley the bastard son of Paul Weller, would be hard pushed to do a 30 minute climb consistently, and I would guess that the differences in each climb would mop up a significant proportion of that supposed gain…
Consistency doesn't matter – the question is, would the lighter wheels give a faster time (e.g. if you did 1000 climbs on each set of wheels with the same rider and bike – theoretical…)