Scott:

**In general, @6500 MSL, @WOT the first 100 RPM reduction gave me about a 7.5% increase in FF, a 1% reduction in TAS and, interestingly, a 90F drop in EGT. **

Certainly you meant a drop in FF, not an increase?   The drop in EGT has little to nothing to do with any mixture change.  As the rpm drops, the pulses of hot gas decrease in number.  This increases the refractory time that the probe sees and it reports a lower average temperature of time.  The pulses of hot gas can be the same temperature, while the reading drops AT THE SAME MIXTURE.

**The next 100 RPM drop (to 2300) had the same FF as 25x2500, with a 3% reduction in TAS (from the 25x25 speed) and, unfortunately, I did not record the EGT.  Looks like the fuel flow went rich (I did not touch the mixture)?**

I doubt the mixture changed much at all.  The EGT reading changed.  See above.
 
**Back at 25x2500, I dropped the MAP 1" and the FF dropped about 8.5%, TAS about .5% and there was a 50F rise in EGT.  The next MAP reduction of 1" (to 23") had a FF drop of 15% (from 25x2500), TAS drop of 3% and a 40F increase in EGT.  Looks like the FF went lean?**

I doubt it.  As the MP drops, the EGT changes even if the mixture stays the same for two reasons. 1) decreased temp of combustion, and 2) the movement of thetaPP away from TDC.  The latter has the greatest effect.
 
**If I were to repeat this experiment, I would re-lean each power change for say, best power at 100F ROP.**

That would be an educational exercise.  I think you will find that the mixture isn't changing very much at all with rpm changes and very little with MP changes ONCE you are out of the enrichment feature range for MP.

**At the throttle reduction from WOT, maybe I should back the throttle down until the MAP just begins to drop, then slowly put the MAP back at the WOT reading without the throttle being firewalled (thus, eliminating auto rich), then lean for best power.  Maybe I should use this as the WOTx2500 base line.  Does this make sense?  Any suggestions?**

You can do that, or you could re-lean between setting changes.
 
Walter