Ehn, I forget the details but it's been readjusted so it doesn't get too crazy. Some math thing that makes it work out, I'unno. Code's somewhere in there. They do stack up to some degree, though.
Now, earlier in T4's development? Passive movement speed shoring up an active buff could literally break the game. Could bring movement cost below zero and actually have you gain energy whenever you move (So you could move 10-20 times, and then get a free attack, and then move 10-20 times again, and...), which was pretty hilarious but hella' broken.
You can really stack up the passive +movespeed these days, though. A lot more item slots offer it. Personally tend to wear feathersteel for a long, looong time, heh. Think my last mindslayer's been wearing the thing since like the pre-teens (and is currently in the forties.).
E: Well. That was... borderline embarrassing. Character couldn't hit prox enough times in a row (Acc was dead even with Prox's defense) to actually bring the bugger's health down.
So, uh. I kited him around a pond until he drowned.
I mean, yeesh. I knew it could happen, sure. I knew killing Prox as a general thing wasn't exactly the hardest thing to do. But killing him without actually being able to damage him? That's a new one for me.
Kinda' hilarious, but being in a situation where you actually resort to that because it's the only way (short of going to another zone and getting some levels, anyway) to take Prox down...