I have a stud SP who is mowing down teams with an economy of pitches.  I have been able to move him up in the rotation several times so far this season (it is fairly early in the year now) and he has about triple the amount of innings pitched as my other starting pitchers at this point.

I am moving him up because he is my SP1 and he is at 100%.....my question is, will this cause a steeper decline in % later in the season? Will he take longer to get back to 100% prior to his scheduled starts because he is logging so many innings early? Is he good to go every time he is at 100% regardless of how often that is?

Seems to me that hitter have a very long damping period, like half a season or more, but pitchers' fatigue is up to date. What you see is what you get. I could be wrong, but that's how it seems.

No. Once he throws a certain # of pitches in the season, he WILL start recovering more slowly. 

Many good owners like to wait one extra day before throwing a guy. So ideally he would go the day after he got back to 100.