205 I regularly roll for over and hour at a time no problem. 
The fights only 3x3 min rounds
I have never fought (so take everything I say with a grain of salt, lol), but a couple years ago was a sparring partner for grappling for a couple of our similarly sized ammy fighters; I came into BJJ with about a year each of wrestling and judo, and I was really surprised how well the pinning transferred over.
Compared to guys who just did BJJ and never spent as much time working on holding pins (which I think translates into a lot of time holding down someone who's trying to get away), I had a much easier time holding mount and side mount and could spend a lot more time holding someone down and throwing strikes.
If you don't already, it might be worth spending some rounds just maintaining top positions after you get there while they frantically try to escape (we did it shark bait style, which is awesome for fight prep IMO).
I've always wondered why the Williams guard isn't used more in MMA.
When wrestlers are in your guard they tend to stay in perfect position to be Williams'd.
It's effective when sweaty (hence its prevalence at ADCC).
It's also a gateway for New York and other 10p clinch techniques and can actually be used to break posture.
Has anyone seen it used or has an idea why it isn't?
People realize unless you are world class, being in the guard is a losing proposition. Every now and then people will get a nice sweep or sub but everyone is so good now and versed in submissions most everyone loses off their back. Even high level guys can struggle off their back against guys at a much lower level. Knowing enough to avoid submissions and punching people in the face negates a lot.
Being on top is absolutely King and critical to winning in MMA, at least at the high levels. 
Yeah I won't be laying on my back with another man between my legs. I'm a republican and we don't do that. Feet on the hips and kick that motherfucker off.
Maybe ask your training partners to try standing up instead of willingly playing guard. It changes guard passing a bit when half your attention has to be on actually keeping the guy down.