I spend a lot of time at low altitudes, towing gliders and in the summer time, this means a lot of cleaning of the leading edges of the prop blades and the wings. Probably not much impact in performance on a draggy power plane, but significant on gliders, as PDR1 mentioned. I remember reading about flight testing, where small squares of masking tape were stuck onto glider leading edges and the polar measured. Degradation was of the order of 10%.
I've had an ASI failure due to a mudwasp nest in a Cessna pitot tube. A tow pilot in my club had an instantaneous ASI failure due to a well-aimed bee strike!
