I've had the car for nigh on 11 months now and I've never had a problem with mis-fires because of water ingress. If you get a mis-fire when it's pissing it down, get a new set of leads and a dizzy cap (Mini mentality there
) as the ones you have aren't doing their job properly.
(Before anyone says anything, YES I have a random mis-fire problem, but it happens in the dry and when the lambda sensor is connected. I've disconnected the sensor and she's fine, even running through puddles (Save for the wheelspin and/or aquaplaning ))
An oil cooler ontop of the engine is a bad idea TBH. You will get flow back when the engine is switched off (Unless you have a check valve in place) causing loss of pressure on restart. Not to mention the extra stress on the oil pump having to pump ~2 litres of extra oil higher than it did before.