A couple things are at work here to reduce recoil so you can watch your critters explode.
First, get a GOOD brake.
By good, I mean a JP tank brake, an MSTN QC Comp or a Miculek Comp.
Then you will want an adjustable gas block or gas tube so you can tune your rifle down to minumum function.
Adjusting the gas is a far better solution than a heavy or rate-reducing buffer.
Heavy buffers work great in duty guns needing ultimate reliability, but adjusting the gas flow is going to do a better job at reducing recoil.
Lastly, you could go to JP's low mass operating system, but on the bench it's probably not necessary.

Quoted: I watched their cool videos and didn't see any difference in muzzle rise between the standard and MGI buffer.

I think there are two different things at work here.
Let's TRY to define them.
Muzzle "Rise" would be the amount that the muzzle jumps during ONE shot fired.
This is what effects AR-15 shooters (semi auto)
Then there is muzzle "Climb" which is a combination of muzzle rise from MULTIPLE shots fired.
What you are getting here is the combined effect of MANY single shots fired.
In the case of full auto guns, the gun MAY not have TIME for the muzzle "Rise" to settle back down before the next round goes off.
This gives you the effect where each subsequent shot is higher and you burst vertical strings of shots.
Now, take a rate-reducing buffer and you get more TIME between shots and therefore less "Climb"
Great benefit to machine guns, not so much effect in semi auto modes of fire.
That's just my take on it.

Also note that the heavier buffer does spread out the TIME that the recoil impulse hits your shoulder.
Therefore a heavy buffer will reduce percieved recoil, but recoil and muzle rise do NOT exactly correlate.
You can reduce recoil with little benefit in muzzle rise.
The AR platform is a particularly good platform for muzzle rise control as the barrel is perfectly in-line with the stock and this design feature greatly helps to reduce muzzle rise.

By properly tuning the amount of gas into the action, I have never found  a need for rate reducing or heavy buffers as they ALL add to the reciprocating mass of the rifle.
The amount of reciprocating mass directly relates to the duration of recoil as well as the felt recoil.