And deployed against primitives. Don't forget that part.

: Like less than 20 feet total. You wouldn't
: be aiming to make a walking tank, more like an on foot attack chopper. The
: ability to change directions within in a couple of steps to keep from
: being hit by vehicle mounted guns and enough firepower to threaten
: infantry and light vehicles… yeah it could be fun.

I can see such a mech dodging Wraith mortars and such.

Heh. When a friend challenged me to rewrite a Legends Episode, one of my original drafts turned the Prototype into such a mech. It mounted a 40mm grenade launcher, a 14.5mm machine gun, and a jump jet that the operator was thoroughly unprepared to use. Would've gotten chewed up by plasma weaponry and relied on surprise and smokescreens to stay alive.

: The closest thing I've seen on screen would be the mech in District 13, but
: without the (currently?) implausibly powerful weapons.

What's so implausible about the weapons?
The deadliness of modern human weapons is an artifact of the cartridge and our own conventions of war. Being the weak, pathetic meatsacks that we are, a lot of ammunition is a compromise between how powerful it is and how much a soldier can carry with him.
Also, we prefer to wound enemy soldiers rather than kill them, because it's more humane and because that wounded soldier requires two more to carry him off the battlefield and treat him.

For aliens that don't care about casualties and have directed energy weapons, the deadliness of the Prawn weapons makes perfect sense.