Technically speaking, you could get away with gigging using just a 5W amp if it was mic'ed up to a PA system. The size of the amp means very little (just take a look at the setup used in arena sized concerts, where relatively small amps can be seen on stage, but each has one or more microphones attached to the speaker grill).

The one setback with a smaller amp is lack of "rumble" - no feel of the air movement from a bigger amp (although your ears would benefit, or at least you wouldn't need earplugs to prevent developing tinnitus)