i also put on a nismo rad cap and i did notice more pressure in the system which has led to my rad leaking . If i put the standard cap back on it dont leak :roll: Sound to me like the water is being forced out of the overflow of the header tank and is flowing down the d/s engine mount and running down the block. If it was your waterpump it would be constantly pissing out imo.
Id say it was likely to be the water pump seems like a common fault.There is a hole in the casing of the pump pointing downwards this is a tell tail for when the seal fails.Mine kept dripping so I got it changed.Put some tissue at the base of the pump casing see if it becomes wet.Not worth continuing to run the car if it is the pump could quickly stuff your car.
caps are rated in psi look at the base of the cap should have no at what pressure the valve opens i think normally 80 psi if any higher you will put stain on any component that can not handle said pressure therefore leaking as a result!
deffo sounds like water pump, you'll probably also find it also leaks worse at idle and hardly at all while driving.
The pump can be changed with the engine in its pretty ackward but not impossible and takes a couple of hours worst bit is scrapping off the old sealant :evil: