Remember that 80% humidity means that the air has 80% of the moisture it can hold at that temperature, not that 80% of the air is water.
Strewth no, that would be sparkling water, not air!
As a newly qualified Puma pilot in the late 1970s I was asked to do some research into this; we had to fly in Belize, in 100 degrees Fahrenheit, 100% humidity. The answer was that high humidiity did have a detrimental effect on DA but nowhere near as much as that of the high OAT. Unfortunately, I no longer have the written info, I think it went into a file on 230 Sqn and has probably gone into "file 13" now.