The XT will probably be fractionally lighter (as in the the weight of two, maybe three Cheddar & Onion Real McCoys) but will also be made from higher quality materials - more metal and less plastic, will have a fractionally more precise action and will, all other things being equal, last longer.
Mechs and shifters aren't 'matched', however, you don't have to use like with like if you don't want to. XT shifters will work slightly more accurately and precisely than SLX. An XT mech weighed against an SLX mech would be the same. An SLX mech will work fine with an XT shifter and vice versa, using XT for both would give you a fractionally(!) better shift and a tiny weight saving, that's all.
The big plastic area you mention, it's not the shim between the mech clamp and the seat tube is it? I only ask, as all Shimano mechs now seem to use that system, so you'll get plastic shims with SLX and XT.