You have touched upon one of the great myths of Aikido - that all of its Senseis are unbeatable martial masters. Truth be told, most of them could not last a single round with a modestly tained boxer from a no-name gym.
Sorry, its the truth. The two things which are never spoken are that it takes so many years to get any good at this art that you find yourself old before you are really competant. The second is that most Sensei are not cross trained with good offensive technique.
Lets take the first - There just aren't any pro sportmen over the age of 40 (other than golfers). Timing, speed, strength - they fade and all the Aikido training won't replace the ravages of time. I know most of the Sensei out there actually believe that they can cause some 25 year old to fly over his hand just by waving it in the air but the truth is in the white belts, isn't it? We do learn an amazing amount of wondrous body mechanics that we then apply to motion and balance and thereby execute 'majic'. But face it - we really like our specially trained ukes...
As for the second - As Aikido teachers we are really limited as to our ability to deliver a body or head breaking strike simply because the average Sensei does not do any work on heavy bags or makiwara. Those who do have a real advantage and may actually be able to defend themselves from a trained attacker. Anyone who has seen a boxer take a solid left hook to the head from someone like Foreman or Ali, or Tyson - and keep fighting - should realize that we mere mortals live in a completely different world from this. And believe it, they could hit even the best Aikidoka.
Against an untrained attacker, well, Aikido is just fine.
The really important thing to consider when dealing with high ranked instructors in not their ability to stop Miker Tyson, but rather their ability to deliver the necessary lessons to the class and make it understandable. A good teacher is like a good coach - the knowledge is there, but the body has begun to fail. Paul , don't mythologize Aikido Teachers, they are rarely able to live up to thier own press. And I include myself.