Cal dominates NCAA Rugby but the University Of British Columbia and University of Victoria in Canada regularly beat them. Checking on Cal rugby web site their results over the last 6 years are not too good against the Canadians. If you go to Victoria's website they claim they are usually the number one North American University team but they do say North American not world.
Seems they trade off with the BC teams for the "World" title.
As for the football issue, I just love messing with ruggers the world over (nearly started a brawl in a bar in Queenstown, NZ with that last comment once). I do still think that a team made up of NFL All Pro running backs, fullbacks, linebackers, cornerbacks, and safeties could become dominant in the world pretty **** quick. They have unbelievable leg strength, quickness, and much better endurance than ruggers give them credit for.
As for the 350 lbs comment, well, there aren't many over 300, but there are a few, and I did say All Stars.
Also, I looked and it seems that Cambridge Rugby does the same thing as its rowing. Whole lot of Cal's best players after graduation went over to play for Cambridge or Oxford while training for the National Team. These then can't be considered college teams, but elite training camps - semi-pro organizations. I'd put money they also have college teams that they field as they do for rowing.