Apologies my original post didnt make sense so I edited it, however that sentence still stands.
Heres a page that touches on this, this lot say 80% though I find that varies a bit.
**broken link removed**
I dont really know how underrating a cap affects it internally, some weird chemical affect on the electrolyte.
The biggest effect is temperature, if you run a cap hot it'll fail quicker, voltage rating has less of an effect.
Edit: heres a cap life calculator, have a fiddle with some values and see:
https://www.illinoiscapacitor.com/tech-center/life-calculators.aspx