Well, frankly, if I had the time to manage it, I'm certain I could still put together a team that could compete for the division or wildcard with as few as 800 IP, in Wrigley or AFC at that...
Knowing that there is a fatigue fix coming out down the pipeline I am less concerned about a minimum IP requirement now. 1,250 does change the common drafting strategies, but may also make ballpark selection more versatile and not as much of a factor in the cookie team process.
I'd personally say if the goal is to have a minimum for the purpose of helping rookie owners try to maintain some level of competition throughout the season, then 900 is a fair minimum that allows that, especially with access to the WW and AAA. If the goal is to further curtail fatigue based strategies or strategies that take advantage of the flaws in pitching fatigue, then I think you have to go with 1,150 or more. 1,150 because with AAA and your 10% (in any park) you are roughly at 1,400 IP for the season. If the goal is to try to balance ballpark use, then 1,250 is perfect. So, depending on the ultimate goal, with the minimum I'd say either 900, 1,150, or 1,250, with the caveat of questioning its need now when there is a fatigue update coming soon enough.