The little plastic overflow valve that came with our kits is way too small to vent the cabin air if the overpressure occurred during full power climb.  It is barely big enough to hand and overpressure situation in cruise conditions.  Besides, unless you calibrated it, you have no idea at what pressure it will decide to open.

As a safety feature, I bought and calibarated a 6 psi pressure differential switch that I mounted under the baggage floor and plumbed through the pressure bulkhead.  I connected it to an unused G900 sensor input called CABIN PRESSURE.  It lights up if the cabin pressure exceeds 6 psi.  If you have another EFIS, you can simply hook it to an idiot light on the dash.

covering the outflow valve seems pretty sketchy to me.   The pressure relief valve is down in the bowl with the outflow valve.  Although John's plane may leak a lot, others may not.