Yeah, the Karma to Hero Point conversion was pretty straight forward and leveraged a lot of the existing rules. (Instead of spending X Karma to increase the color of the result, you just spend 1 hero point, etc.)
For advancement, it was literally 1-for-1, but I only allowed the standard ranks. (So if you banked 10 points, you could raise your Strength from Good (10) to Excellent (20).) I found that over the life of that game, it wasn't that disruptive, but you might want some guidelines on how far certain abilities can be increased. (Human limits as well as caps on offensive and defensive powers.)
And when I say "modeling rules" I think I mean the same thing you do. I sit down with the players and say "What do you want your character to do?" and we choose and detail out appropriate powers and effects. We then use benchmarks in the existing game to determine appropriate ranks.
, but I have a good relationship with my players and I run superhero games so that characters are always matched up against villains of equal power (so the Superman character has a Bizarro to fight while Batman is scrapping with ninjas).