There are definitely good luigi's out there. The only thing is that players who have good luigi's usually have better pika's, kirby's, fox's, falcon's, mario's, yoshi's, dk's, puff's, and sometimes ness/link and even samus. At the highest level the game really boils down to who can penetrate the others defenses (or who can wait long enough for the other to fail lol), and luigi gets ****ed in that area. I don't really understand what you mean when you say you use luigi's slowness to your advantage. Elaborate?
Also, I feel safe in saying Bcow has played some of the best luigi's in NA, but he's also played some of the best top tier characters and would much prefer to play against a good luigi than a good pika/kirby/fox/falcon, as would pretty much everyone.