Interesting if true. Maybe Ford wanted the cars to go into the hands of collectors, not people just looking for better performance. So they announce the performance gains after the majority of the cars have been sold.
I'm still very skeptical though.
Can you reference the times you are talking about? Motor Trend says the 0-60 on the 2015 GT is 4.4 sec, 2015 i4 is 5.6 sec. The 0-60 on the 2005 GT is 5.1 sec. 5.1 sec vs 4.4 sec is a fairly big difference.