IMV, the elephant in the room is that the rocker cover should be in slight vacuum from the breather to the inlet manifold. I suspect what really happens is that the engines are old and the rings are allowing excessive blow by, and at high power this pressurises the crank case and lifts the rocker cover seals, allowing oil to seep/blow out. If the rocker were in vacuum, that wouldn't happen.
I've also heard it said that replacing the rocker covers with metal ones can cause the crank seal to start leaking. Again that can only happen if the crank case is under pressure, and the only place for positive pressure to come from is piston blow by. If the breathers can't cope with the amount of blow-by, then you are going to end up with a pressurised rocker cover, crank case and sump. Reinforcing the rocker covers with metal ones gives those seals enough strength to retain the pressure, so the next weakest thing lets go (the crank seal).
So yes it is important to make sure the breathers are clean, but IMHO that isn't the root cause of the warped/leaking rocker cover issue. Engine wear is. Unfortunatley the only way to know for sure is a complete engine strip down and cylinder bore/piston/ring work. Difficult to justify on a sub £1000 car.