Battery Voltage

Is this normal ? .... Dropping battery voltage.

I would expect the voltage to be steady.

Possible issues may be

An increasing electrical draw with rpm - possibly due to coolant pumps, fuel pumps, coils and injectors?

Slipping drive belt - this can be due to the load increasing with rpm and/or centrifugal force reducing the 'wrap' force holding the belt to the pulley(ies). The poly-V type tends to need to be tighter than the old style belts, which can still slip, too. Oh, of mixing parts from different vehicles, there may be a slight incompatability in the belts - this is really restricted to the single, deep V type, though.

Something else?

I've seen alternators wired to disable the field voltage when the engine at at WOT. So you never charge while at 100% throttle, and thus have maximum output power with no load from the alternator. Is this a race car?

Good point, but it was charging with a gradual drop off, and still above battery voltage - assuming 12V and not 16V system, of course.