I think that any style can be effective BUT it requires the practitioner to "master" it. I don't mean become a recognized Master..I mean to learn the system and make it become second nature. For example:
I think that Mike Tyson could whip the average Karate or Kung Fu practition found in your average school any day. But I think that Bruce Lee would have killed Mike Tyson in a fight..and I think that Chuck Norris could kill him too.
Royce Gracie is a grappler...but he has beaten men of nearly every style in the UFC fights.
Why? Because these men have mastered their styles. A highschool wrestler could beat the average guy on the street...but most likely could not beat Royce Gracie (or Chuck Norris for that matter).
I think that any system which is well rounded and has some serious techniques can be deadly effective in the hands (and feet) some someone who has seriously learned the style and has "mastered" it in him/her self.
As far as the AVERAGE wrestler and the AVERAGE martial artist (by the way..I'm of the opinion that both boxing and wrestling are martial arts..but that's a personal view)...well...that depends is the "average" martial artist either knows how to fight on the ground..or knows how to cosistently avoid going to the ground.
My boxing instructor (Western "American" boxing..not Chinese) had been in many a bar fight and even was attacked with a knife. Unarmed against a knife, he literally bunched the assailants left eye right out of his head. He had been attacked by grapplers and could punch so well that he was never taken to the ground...ever. He could knock out all the grapplers who came in close enough to try to get him (punch in the jaw or temple). He was the heavyweight state champ of Geogia for 6 years (back in the 60's). Willie Jay Johnson Jr. Was he the "average" boxer? well, maybe not. But I think the point is...a wrestler can beat anyone and anyone can beat a wrestler...it just depends who's faster and smarter, I guess.
And, yes, being is great shape is very important.