after playing a bit more, and heading to various tournaments over the weekend, I will say that the default side b is still ****. It cost me a GFs set. BUT!!!!!! despite all its headaches, it does its job. It's still a command grab, and it's still a move that kills VERY early (even without rage!) I think I'm starting to figure out the proper ranges for it, and now that we have more control over the move, it is a bit more satisfactory to use. This brings it from a 1 in happiness ratings to...a 2.5.
but that's not what this thread is about.
I'm not sure what I think about Dash Slam. It's really awkward to use in the air because there is a brief pause before he actually moves, but it is a mobile command grab so I guess that's cool for what it's worth. To be honest though, you have to weigh your options. If you're thinking of using Dash Slam, you DO still have a dash grab and a pivot grab, which after a pummel & throw will come close to matching/exceeding the reduced damage of dash slam at the cost of just playing neutral a bit differently (it's really not that hard). You may be better off either giving yourself more mobility/a "combo" game with Dash Slash, or giving yourself a guaranteed kill move by sticking with the default side-b.
Don't forget, with each use of Dash Slam, you are throwing yourself at your opponent in a game where grabs do not have armor. Think about that for a second. Our pivot grab is already great at scooping opponents, especially airborne ones, and doesn't really suffer from the "****, I have to worry about trades" problem due to its disjoint aside from dash attacks.
Dash Slash is good, we all know that.
The beauty of Smash is that most sets involve anywhere between 2-5 games. Analyze your opponent! If you see that your opponent is scared of you and is prone to shielding, or is the kind of player that uses grabs from a defensive manner (ie shieldgrabs, which a lot of newer Smash 4 players do too much of) set your Bowser up with a command grab! If he likes playing the zoning game or waits for you to over-commit, play it back with him by adding a mobile, combo-based threat to your game. You're never stuck to one option per set, and sometimes Vanilla Bowser is just fine.