Most Well Known American Manga-Ka?

In terms of manga-ka that emerged from America, there's Me, Piro of Megatokyo, and Nyanko-Chan. Maybe Felipe Smith, Svetlana, Adam Warren, Bryan Lee O'Malley, and Jared Hodges. I'd say readers know who we are. Everyone else is a virtual unknown. Publishing in print and webcomics is sort of a rite-of-passage for American manga-ka. 
Much as I LOVE character design and costume design. Story and Action/Adventure as well as the intensity of a comic's artwork can define a creator's work's success as well. Action scenes that look cool and fast motion that doesn't look cliche is one of the hardest things to draw in manga. Master those things and you'll be well on your way. Just don't expect to master them on the first attempt. Or overnight. Developing your sequential art skills takes time. At least 2 to 3 years of 100s of BAD comic book pages no one in the public domain sees. That's what I did that I never bring up. Sure, online it LOOKS LIKE I only drew five pages. In reality I drew 150 SEQUENTIAL ART pages by hand! For mosts artists that are really good, your Dave Sim's and Todd McFarlane's, your bad art is going to outnumber your good drawings 10 to 1. 
I think the 10,000 Hours rule ESPECIALLY applies to manga. Not just some of the time. The vast majority of it. I still haven't put in my 10,000 hours or 10 YEARS of drawing manga. I'm a lot closer than I was though. I've been drawing manga since 2004.