I've been made to re-pack my hand baggage at SVQ because it weighed 11kg (and it was well under size physically), and my mother has recently been charged excess baggage at East Midlands airport for having a check-in bag weighing 16.5 kg instead of 15kg.
It is impossible to predict what will happen on any given flight but Ryanair are certainly not afraid to impose a very strict implemenation of their rules when it suits them.
I would say though that most of the checks seem to take place at check in, and boarding is usually such a scrum that they don't have time to inspect hand luggage at the gate.
The biggest risk is that as Ryanair don't gate-check bags, if they did decide to get picky with your bags at boarding time, checking in your outsize cabin bag will no longer be an option and you will be off loaded from the flight.
