I don't see why she wouldn't be amazing in 2v2 either. The reason Sly is so great is that he is so hard to hit, well, Kat is even safer than Sly, can burst very quickly, has an unpunishable level 1 and is great for team confirms since she can keep the partner so close to the ground during combos. Her great mobility means she can dart around the place, avoid getting hit, bail out her partner using debris (Sly's biggest problem in 2v2 is that he isn't great at saving partners). This is all in theory yes, but I see little reason as to why she wouldn't be one of if not the best.
It's great you said it was all in theory because I could detect many things wrong with this. Albiet Kat is great at 2 vs2 she isn't the best by any means. This is why players like iiGGYxD and myself feel she should be legal in offline 2 vs 2. It's because she has no glitches (which she was orginally banned for) and she isn't the best. That being said, there rarely offline 2's so it's not that big of a deal.