Exhaust sleeves help evacuate gasses from comb chamber due to good geometry and velocity effect. I've read that flow bench testing shows 'em to be very good flowing up to a point somehwere around 300 hp on L6 engine. So if you're trying to beat that limit w/ n/a engine then consider other head options - also review your pocketbook and whether there's any screws loose upstairs.
IMHO the best head is one you already got if it ain't cracked or warped, and it matches your exhaust manifold. Maybe if you're in the league with Dave Rebello or Sharpe Racing, you might need to worry about which head is better, otherwise, there's lots of other things: cam, ignition, carburetion, etc. that's gonna make more difference.