I have to chime in with SNS3 (love reading your posts on technical matters) and 411A, and especially with the seemingly much overlooked one line post by werbil a way back.
No combustion -> No pressure increase in the cylinder -> less exhaust backpressure -> less remaining pressure in the cylinder as the exhaust valve closes -> less pressure in the cylinder as the intake valve opens -> more air into the cylinder -> drop in MAP, even with everything else remaining constant.
The effect would be there. I have no expertise or experience which enables me to tell if it would ever be noticeable, so there I'll have to trust those who claim it will be.
Exhaust backpressure is often overlooked but can still have some very interesting effects on the performance and behaviour of piston engines.