Rule of thumb every lb of increased pressure the boiling point is raised 3 deg F. The cap is pressure relief fail safe, if you are blowing off a 20lb cap your operating temperature reached the 280 deg F range, that is way too hot IMO and puts a lot of stress on the components (as you have experienced).
However, the fact that it actually got this high indicates the cooling system is not functioning well and with a lower pressure cap it would have still happened, just sooner! There is a fundamental issue with your cooling system, wrong thermostat or fans not coming on correctly. Is the cap you have the correct one for the tank neck length? It might also be that your gauge is reading inaccurately if it wasn't ready at the high extreme (260F gauge?) temperature prior to boiling over.
Silicone hoses require specific style clamps to prevent shearing or extruding into the hose. Personally I stay with good quality (Gates) rubber.
Were you stuck in traffic and engine idling i.e. the water pump flowing at it's minimum and not efficient?