And BTW if you're at 85% uptake capacity it absolutely helps to train that up to 90%. It's the holy grail of endurance training in fact, and to me it's ludicrous to claim that it's irrelevant because we use less than VO2max at FTP. A completely irrelevant point, and even less relevant if your hour performance is limited by getting winded. Less relevant still if you don't care at all about your hour performance but you like to ride fast. Or stronger up hills, like the topical question.
If you can gain that 5% by increasing your oxygen uptake, or by increasing your efficiency, it's still 5%.
One big reason you don't ride around - or race as someone inevitably brings up - at 60 rpm is after training at higher RPM the efficiency differences diminish. Training specificity. Regardless of one's maximal oxygen uptake we prefer the higher cadence because among other things we find velocity changes to be less fatiguing.