No, dude. It seriously goes as fast as we want it too. The problem is that squishy meat bodies can't handle the acceleration and associated forces.
I thought aerodynamics also came into play at least a bit so far as top speed is concerned? Regardless, with a mech we have additional constraints that stop us reaching full speed.
1. Trailing limbs can smash buildings into pieces (and kill the occupants)
2. Big bulky body makes it hard to maneuver around squishy human bystanders
3. Extremely un-aerodynamic.
Aside from that, meguca seeing a massive mech are naturally going to want to keep their distance, possibly keeping them out of gem yank range. The MOF is a far less intimidating shape, allowing us to get into range without them fully realising the danger. Being knocked into a building (and destroying it killing bystanders ect ect) is also fairly inevitable in mecha combat. Against meguca a mech is not a terribly efficient way to do things compared to our other options. It does, however, have other advantages:
1. Intimidating. We may not actually use it in a fight, but having a mech looming behind us can do wonders for ensuring smooth negotiations. As the saying goes, speak softly and carry a big stick.
2. Weighty. It's less likely to get batted about by large impacts, because we can brace the thing and it'll likely be larger than a MOF.
That's why I think having a mech would come in handy, but not in actual combat against meguca. We could possibly practice using one inside the next witches barrier.
Edit: Anywho, why
don't we just say we went over and beat everyone up until they stopped fighting? I'm pretty sure Kyouko would only respect us more for it.
"Oh, yeah, about the gang war, Mami and I (and Kirika) went and beat up everybody until they agreed to stop fighting, so that's dealt with. Well, we tried to talk to them first, make them see there was no reason for them to fight each other but they were being unreasonable so we beat them into a pulp."
[jk] So yeah, we joined up with the losing side,
captured all of the winning side and imposed our dominion upon the FOOLS WHO DARED TO DEFY OUR WILL! WE HAVE ACHIEVED PEACE THROUGH OVERWHELMING FIREPOWER!