I voted for medium strength. Here's why.
This is not proscription, just observation based on years of play. This is the balance I think works best to keep Mary Suedom in check but let us have tons of fun. Also, when I say expert, or master, or average, I'm noting discernible levels of experience, skill, ability and talent among the various aspects of life in the game.
New characters should usually start out as average with potential. Usually, not always. Why? Because 90% of the fun lies in the journey. Take my first character. His first combat experience was against Unar. If you don't know who Unar is, let's just say that she was the greatest bounty hunter ever seen in the game. Hands down. I am not exaggerating. Although it was a different play-style then, I'll admit. My Ylian was obliterated. I learned. I became her apprentice, of sorts. It built gradually from there. And it was a blast. Scraping for trias, building skills, fighting as a mercenary, and doing tavern RP for a couple hours a day sometimes. Amazing gaming experience. There's my anecdotal evidence.
There ought to be masters, but they should only be a small proportion of the population. Mastery is lifelong, and becomes an art form almost always. It consumes vast amounts of time, so much so that other skills, talents and aspects of life are left wanting. That is the sacrifice. Unless your character is blessed by the gods with exceptionally long life or supreme talent, then this remains the way it is. Let me say, for a second, that you might think this a 'realism' argument -- but its not. I am using a fact of our lives, and noting its usefulness and importance in a role-playing community as a measure of fun in achievement and keeping a lid on Mary Sue explosions. These explosions happen, and they ruin fun. (See argument: extremes suck).
I used to believe matching mechanics (i.e an actual 100 dagger) with RP stats was the best most balanced route. Then I realized I had a life, and so did everyone else -- thus it doesn't work that way usually. Although I used to enjoy building skills, now I don't do it at all, almost entirely because of how much time my studies consume. I want to login, role-play, have a bunch of fun, and bounce out. I believe this is the case for most of us.
I swear, I'm not making a realism argument. I am taking realism and applying it here for the purpose of balance. There is a distinction. Oh, and I take no issue with making a super powerful character as long as its earned to a large extent and consented by those around you. A villain whose schemes sweep across the world would be an example where this could be fine.
P.S. Kull was my second character. He is three, possibly four haha, years old IRL. He started out average with potential, a fresh adult, and has become a powerful fighter. He earned that -- through role-play. He loses, and wins, and will likely die a true death within a year given his path. I'm not planning that -- it's just likely. So there you have it -- you can play a villain, who earns his power, have tons of fun, and be fine with him or her losing and eventually dying off.