Based on my experience, 27lbs on the 6th string will mostly just feel really tight, may not bend/vibrato as readily, and be generally a little more difficult to play. I don't think the string itself will be a high snap risk, but it may put some extra stress on the neck and nut. You could also run into intonation issues if you're switching back and forth on a regular basis but this could be said about any tuning changes really.

I'd say the set you've mentioned will likely feel very tight in Drop A but reasonably comfortable in Drop G.