I have a habit of spamming Spring Jump as a "GTFO" move. Whether it is his best or not is unknown to me, but it certainly gets the job done. If you want fastest actions from a ground release you're going to want Jab as it comes out on Frame 3, maybe a shield/spotdodge since Jab has pathedic range/is just a terrible move overall (I wonder if grounded Spring Jump would get the job done...). I believe this is where prediction might determine how things go if it's a ground-break.
If I have this straight both Bowser and Sonic can act on frame 30 should Sonic ground-break, meaning if Bowsers tries to do a grab Sonic needs to do something that gives him the frame advantage, which IIRC is only Jab/Shield/Spotdodge since Bowser's grab (or just any non-tethers' grab) takes 6 frames, right? This is only grab, I'm not sure if Bowser has anything faster than 6 frames but this is where your knowledge on your character comes in. I don't have the frame data of things like tilts/Spring down (although I', fairly certain that Spring Jump is fast) down but if you look around the board you'll find a couple of things. There might also be some additional misc info on the tactical boards like spotdodge/rolldodge data and all that jazz.
However if this is too much trouble you don't have to do it, I would appreciate it but I won't hold a grudge if you say no.
Also you're right on how Grab-releases work. Except that if a characters feet do not touch the ground, for example Jigglypuff, they are forced to air-break unless you get them into pummel hitstun. If their feet DO touch the ground then unless they initiated a jump or whatever they will always break loose to the ground.