Some people feel that all they do is hold in heat and make the car slower because the turbo is actually hotter other's feel that they help to protect the paint and don't make the car slower.
Mine has an aftermarket hood blanket which I think will protect the hood from the heat so I am leaving mine off.
I havent got around to cutting mine since putting on the THDP. I personally feel, with the stock turbo anyway, that the shield makes the engine bay look cleaner and less cluttered. Maybe with a brand new non stock appearing turbo it'd look neater without the shield but 'til i purchase one i think i'll put mine back on.