I sent an email to Mercury Magnetics asking about their transformer for the AC4TV and here's the response:
Thanks for emailing me at Mercury Magnetics.
"The Mercury Magnetics AC4 output in the kit has an approximate 9500 ohm primary impedance. The stock one is lower (as I recall it is around 8k).
The combination of the Mercury Magnetics upgrade PT, the kit parts, the circuit change and the OT are what make that higher primary impedance work so well.
I'm surprised their transformer has a higher primary impedance than the stock transformer. I don't know what the reasoning is behind that.