Do you mean spinning out, or just that you felt you could have gone faster on a bigger gear? Not sure what cadence you think counts as spinning out, but 150rpm on 50/12 is 50mph. You do know that you can only save 12s/mile by going 10mph faster than that, don't you?
You don't have to be doing 32mph+ for 10 miles to justify a taller gear. If you touch that at all and you're under geared you're missing out.
I'm pretty slow but was spinning out very easily on a (windy) rolling course yesterday on 50x12t.