Look mate, I'm not saying the 400 is the end all be all of saws cause it not. But your experience with the one you have, is pretty against what most of us have experienced. I primarily cut hard wood, actually very little soft wood. Idk how long a bar you toss on it, the 400 is the better saw. Yes against the 562xp, it's a faster saw, against the 620p yep its the faster saw. Been there done that, and any performance mods just makes the gap even farther apart. Actually mine darn near keeps up with my logging buddies 572xp With just a bark box on it. Funny part is the last time I was working with him, he kept grabbing the 400 over the 572xp. Lighter and easier handling, even if it was down a little in power by comparison.
I also don't understand you obsession with a 28" bar in hard wood. None of the saws you mentioned will pull that size of a bar worth a crap in any real hardwood.