I go back to my word-problems... If 2 trains leave the same location at the same time, traveling the same speed and the destination is the same distance...
Practically speaking, the shortest distance between your [fist] and your partner is a straight line. Presumably, his attack will be on that straight line. You simply cannot move a longer distance than your partner without changing the equation. For either partner, you can change distance, speed, and duration.
If my irrimi abdicates the line, then I necessarily must either negatively affect my partner or speed up my movement. If my irrimi closes the critical distance while establishing a new line of attack, I will actually reach my target first without altering the remainder of the equation.
While it may look the same, starting my movement sooner than my partner is not sen sen no sen timing, it is compensatory timing. Here's what gets me... Clark Sensei said it... sometimes the timing is evasion. I have been racking my brains for several years now to reconcile that little gem. I think there is a difference between the defined timing of combat and the observation of compensatory timing. Best I can describe it... Nolan Ryan (best pitcher, ever) could throw a fastball 95 mph+. As a non-professional hitter, I could hit a fastball if I started my swing mechanics timing sooner. As a professional hitter, I could hit a fastball without the need to start my swing earlier. Things get tricky as soon as I need to first determine whether to swing or not, then figure out where the ball is going to be. Sometimes we need evasion to buy us time to act... That's OK, but I am not sure that timing actually is combat timing.
I think this is why in the pressure cooker of intensity, irrimi is is tough to do. I think many of us mask our bad combat timing with compensatory timing. As soon as our partner speeds up though we cannot match him. Take away the foreknowledge of what is going to happen... Well, our partners obviously have bad energy, or didn't do something right, or aren't "committing" to the act. or whatever.
As a wrap-up to kevin's question, I think that sen sen no sen timing is performed under the intention to supersede the attack and encourage an alternate cognitive thought process. I think simply moving into a new position that your partner can [still] attack has nothing to do with the timing of combat (or distance).
This may be just word definition differences but the point of starting movement before the partner not being sen sen no sen to me obviously is. As I said, it may be differences of definitions.
Another point I would like to make is that good irimi is also a matter of sen no sen.
The compensatory timing as you call it I would say comes under go no sen, moving after the attack has started.
Simply put I would say sen no sen is moving before and sen sen no sen is presenting a target which makes the other attack, thus you are controlling the whole situation.
As you used a baseball example I'd like to use a tennis one, professional tennis, the top boys especially.
When you think how fast the balls are moving and how far away from the opponent they are heading at speed then you can see that mechanically trying to work it out and move is far too slow. Thus as I've said before when on form they are considered to be in the zone. They are tuned into each other and going with feeling rather than analytical thought. They feel and 'instictively' know where that ball is about to be hit and are already on their way there. This is an example of sen no sen in tennis as I see it.
Irimi is a path the way I describe it and thus if you use sen no sen and move on that path then you have closed the space down and taken over before the opponent has finished what he was trying to do. That's high level of course but nonetheless real.
Now not that that is high enough O'Sensei said that you can go even beyond that and seemed to imply that there lay true Aikido. Small steps long journey ha, ha.