The sleeve isn't that heavy, but it is easier portable to go non-sleeved. A shirt clip helps, too. I would burn in the cable when you get it, too. It'll sound a little darkish at first, but opens up after a bit. If I were you and you were willing to spend $200, I'd get the SPC cable used and the Midnight. Compare them, see what you prefer.
I think it just enhances what the SM3 does already. I hate to use this term "refine," but in this case I believe it applies- that it refines the sound of the SM3. It sounds more dynamic and clearer. What it doesn't do is make the sound signature sound thinner and unnaturally brighter like the Starlight did. The Starlight was a better match for the UM3x and my old Westone custom the ES2.
I had the C5, and after about 4 months with it I decided the forward mids weren't to my liking with the SM3. For me, it wasn't a perfect match but that's personal preference, same as everything else. I like the Leckerton UHA-4 (with AD8620) and the UHA-6S MK II ( AD797 or ADA-4627) low gain version. The UHA-4 is smaller and more compact but may be harder to find. The 6s MK II is probably more available because a new model is coming out. Both sound real good but the 6S MK II is a step up sonically. It's more powerful and adds more soundstage and depth, but it's a bit bigger and heavier, but definitely still portable ( I use either of them at the gym in my pocket strapped to my IPod Classic). A lot of SM3 owners don't use an amp, the rationale being it doesn't need one. I don't think it needs one either, but an amp can definitely add something, like that last 15-20% that for me, makes it worth it.