How significant is a 5-10 mb drop in pressure over a 20 minute period?
On the synoptic scale (thinking along the lines of cyclones, weather fronts etc.), then that's a pretty significant drop in that amount of time.

A bomb or explosive cycogenesis in meteorological terms is defined as a drop of at least 24mb in 24 hours, so your observation easily fits this description of being a significant rapid pressure change. There's a list of similar events here.

You ask what the significance would be at the surface, and only 80m up, which makes me think you might be talking about smaller scales (perhaps?), in which case @naught101 is right in suspecting small scale eddies from buildings could be causing the surprising readings. 80m isn't really going change the significance of an observation that much on synoptic/meso -scales.

Sources:

http://glossary.ametsoc.org/wiki/Bomb

Synoptic-Dynamic Climatology of the "Bomb"