id leave the sleeve in but dremel down those two nubs that held the flap in place. I could see it having sealing issues without.
id be willing to bet that the inside of the sleeve matches up well with the port on the head, where the unsleeved one wont and looks bad for flow (sharp edges, weird shape)
==2012 Kona Blue Mustang V6==
==1995 Light Iris Neon Coupe==
==2005 Sangria Red ZXW==
Sold but not forgotten: 2001 ZX3, 2007 ZX3, 1972 Skylark