BIK
As I said in my post my observations have been based on the different Boeing types I have flown over the years.
As far as your question on fuel control units yes I guess it's entirely plausible that a fuel control unit could be produced to counteract this reduction. While I could see the possible merits in a control unit that maintained a constant EPR why would you design a fuel control unit that actually increases the EPR on a take off roll ? Wouldn't this system increase the stresses on an engine as your speed increased ? (not a very good combination).
It would make more sense to me to allow for the reduction in EPR in your take off performance calculations rather than
A> Go to the bother of designing a fuel control unit that is going to increase EPR as the take off roll continues and
B> Design a system that will increase the stresses on the engine during the more critical phase (high speed) of an already critical procedure (take off).