Tried this yesterday.
190 kts IAS, 2000', 2400 rpm (constant speed prop), 36lph (50 deg Lean of peak on richest cylinder). IO360.
When I turned one of the mags off the engine when very rough the EGT started to rapidly rise. I did not leave it running on one mag for more than a few seconds because of how rough it got, Both Mags behaved the same.
So for me if I loose a mag I will notice straight away.
Not what I expected.