Myofibrillar and Sarcoplasmic hypertrophy do not lend themselves to being a black and white "it's either one or the other" result. Depending on your initial muscle mass levels, you may put on mass which will be sustained simply because it's muscle you should have had to begin with. In that case, putting on mass would not mean it's sarcoplasmic. Nor does it have to be 100% Myofibril density increase. It can be a combo which favors one or the other.
As far as the benefit of mass goes, let me first correct the previous poster on his nomenclature;
There is no such thing as an 'Olympic Powerlifter'. It's either Olympic Weightlifter OR Powerlifter. Powerlifting is not an olympic event, nor are the two disciplines the same in the execution of their respective lifts. A powerlifter squats, bench presses, and deadlifts, competing in one of many federations at state, regional, national, or world level. An olympic weightlifter snatches (not a power snatch - a full snatch) and clean and jerks (not a power clean - a full clean and jerk), competing under ONE ruling body (the International Weightlifting Federation) and ultimately the goal is to compete in the olympics (if your lifting weights to be a weightlifter and not a just a fitness enthusiast). Strongman Competition I'm not as clear on in terms of ruling bodies. They were under 'Worlds Strongest Man" but that may have changed, and there may be more than one now. Anyway, strongmen compete in a whole variety of odd object lifting, truck pulling, and grip / carrying feats.
Enough on that.
I am a competitive powerlifter in the American Powerlifting Federation in the 242 lb weight class. My opinion is that training for hypertrophy is NOT critical when you are trying to improve your power lifts. It would stand to reason that this is true of olympic weightlifting also. The main reason why is that these are weight class sports. Thus, the stronger you are, while being as light as you can be will be to your benefit. Let me use an example;
The weight class below me is 220 lbs. I lift in the 242 lb class because I actually weigh 233 lbs. So let's say another lifter weighs 239 lbs. Both of us compete at 242 and we achieve the same three lift total of 1,500 lbs. I will win because I am actually lighter even though we are both in the 242 class. Excess fat is obviously the worst. But excessive hypertrophy can be bad too.
There are tons of examples of people who are super strong in powerlifting and very light relative to what they lift.
Bob Peoples deadlifted 725 at 180 lbs of bodyweight.
Lamar Gant deadlifted 688 at 132 lbs of bodyweight, squatted 595 and benched 352.
Dr. Fred Hatfield has a whole list of his accomplishments at various body weights here:
Rock Lewis benched 600 lbs raw in July 2007 at 241 lbs of bodyweight.
Brian Cass has all three of his lifts over 700 lbs at a bodyweight in the 220's.
It goes on and on... John Inzer, Eric Cressey, ...
Getting bigger DOES NOT necessarily mean getting stronger. And training for hypertrophy specifically is a waste of time in my opinion.
Just be careful when you research this because their are guys 275 lbs and over who squat and deadlift in the 900 lbs and some over 1000, as well as some ultra heavy bench presses. You have to pay attention to whether they are using gear or not - by that I mean squat suits and deadlift suits and bench shirts. The geared (or equipped) bench is over a grand already. The raw world record is 715 lbs. So make sure you know what you're reading before you make conclusions. All the lifts above, with the exception of some of Hatfields lifts, are all raw.