at what point does one of these become neccessary?

i have no boost gauge on my car yet, but according to the previous owner, who had it set up by power engineering it is approx 20psi

stage 3 making 311bhp, does this sound right?

i really like the look of a pro alloy 500 cooler for my 4x4, just unsure as to how neccesary it is

what do you think, should i go for it or not? any wiser ways of spending my money, other than a boost gauge