You'd need a lot of neg. camber to start hurting the handling gain.. like 6 or more degrees. That's basically not possible on an EF, unless it's literally sitting on the ground.

If you insist on a camber kit, a complete upper arm is the way to go, front and rear. Omni, Skunk2 and King Motorsports make adjustable rear upper arms. A cheaper rear camber kit is some longer bolts and washers to space the rear upper arm out from the body more.