Penn was only around 150? That's even more impressive.
I'm intrigued by the hypothetical GSP versus Mariusz fight even more after reading your post.
I know better than to ask about a hypothetical strongman...I mentioned Mariusz because he actually has some fight training.
I had no idea that GSP got schooled in Abu Dhabi. I guess that's the thing about MMA fighters - they are generally not the strongest in multiple areas; just well-rounded.
John Danaher said on the Joe Rogan podcast that GSP has been submitting some high-level Jiu-jitsu fighters in the lead-up to the Bisping fight.
He didn't mention any names though.
Yes, there's a difference between high-level Jiu-Jitsu players, those that are standouts in a particular school and local tournaments, and world class competitors from all around the world. Abu Dhabi is like the Mr. Olympia in no gi submission wrestling. Not only do you have to establish your name to get an invitation by winning a major tournament you still have to go through the Abu Dhabi trials.
It is worth noting that of the elite, the best of the best, world class Jiu-Jitsu players with a few exceptions have not fared that well in modern MMA. The few are Jacare, Demian Maia, Fabricio Verdum, and recently, Raphael LoVato Jr. who just won the middle weight title in Bellator over Gegard Mousasi. The two considered the best of the best in world, head and shoulders above the rest, Roger Gracie and Marcello Garcia, tried their hand at MMA and wisely decided to stick to Jiu-Jitsu.
I think a D-1 wrestler is more combat suited than a Jiu-Jitsu player even though Jiu-Jitsu is more combat-oriented because of submissions. All else being equal, taking a wrestler and teaching him BJJ and striking will do better than taking a BBJ Black Belt and teaching him wrestling and striking.
Tournament Jiu-Jitsu where you are wearing a gi is very technical and ofter slow-moving. A lot, if not most, of high level BJJ comes from the guard. Because you can use both legs and arms there are more submission opportunities while you are on your back than the top position and many prefer fighting off their back. Well, being on you back when punches are factored into the equation is pretty much not a good position to be. In a tournament, you have to break free, i.e., "pass" the guard before the fight can continue or you will be penalized for stalling. In MMA there is no imperative to pass the guard when you can just sit there and keep dropping bombs for five minutes racking up points or getting a stoppage or knock out.
Also, wrestlers are more aggressive, their takedowns are more practical and on a higher level than BJJ, they can control you better with their bodyweight whereas in BJJ they use the gi to control you (no gi in MMA), and wrestlers are generally in better shape, more accustom to maintaining a high level of intensity for longer periods, and tougher mentally.
Again this is just a generalization stemming on how both disciplines are run. Wrestling is run solely as a sport catering solely to high-level athletes. So you are just naturally in a higher-level environment and competition and if you don't perform its "Bub Bye". It's like being in a calculus math class at a State college and a calculus class at Cal Tech or MIT. Jiu-Jitsu, on the other hand, is generally run primarily as a business first and there is zero danger of being "cut" from the team because you suck. D-1 college wrestling is brutal and many who would normally excel in BJJ would break mentally in a D-1 training camp.