Shroud DOES affect it. Use a shroud! It forces the air through the exchanger. If possible block other free/empty space on the sides etc. as well. It won't adversely affect your open road driving, at least not to a point of concern. If you were really worried about drag co-efficient you'd not have started with a Lancer.
Generally you'll want your shroud to be angled slightly to direct the air towards the fan also, I guess that comes down to how much room you have though really.
Do your radiator/cooling tubes flow past the exh. manifold side? If so, try heat wrapping on both the turbo and the fluid pipes. Alternatively directly flow further from the turbo.