Shutting the alternator off will drop the voltage to battery voltage 12v it's only 13-14 volt when alternator is charging under load

 

And a ballast resistor type coil only gives

12v(battery voltage) on cranking and drops to about 9v when running so whether you disconnect the alternator or not you will still have 9v coil out put you can bypass the ballast but will burn coils out fairly quickly

Just a little food for thought after disconnecting the alternator you'll drop battery voltage and when it reconnects the alternator it will charge it's ass off to bring it back up if it drops to much voltage which will mean more load on your engine so I'm not to sure it will be all that affective may have to put that into consideration aswell

The Kick down switch seems the way to go.

Use that to disconnect the field winding to ensure to alt is 100% disconneted.

Need to swap the alternator for one with an external reg.

Prior to doing any permanent wiring, we have some dyno time booked in, I'll do two dyno runs, both at full electrical load.

One with the alternator charging and the other with the field winding disconnected.

 

I'll let you know how I go.