Don't get this oil and air flow meter problem, how much oil are you splashing on it. I have pipercross and k&n fitted to my vehicles my sons have pipercross fitted to both their's and I doubt if there is a teenager out there who has not replaced the standard one with an aftermarket one. My friend has a v8 running on gas and he has done the same. The outer foam gets oiled and it should not be leaking past the inner filter.
Does it make a difference, a little bit on a standard engine but a damn sight cheaper in the long term and less maintenance. 100,000 miles they quote before a clean, but I clean mine more often, and properly.
It also looks better and free's up some space.
you wont be able to wade with it obviously but the ralley boys thrash through water without much problems, and it's unlikely that it will suck water unless you submerge it.
If you are going to buy one go to the breakers first, lots of boy racer cars and you can get them for a fiver, or buy new for 40 quid upwards.