Multiplexing accuracy

[Rig Wrench]

How much of a discrepancy is in factory multiplexed gauges and regular old mechanical? I've read on hear that the factory boost gauges can be off, but what about others?
The reason I ask is I've been battling a high egt issue with a cm870 565 for a while now, I'm staring to think I may not actually have one. I can get the gauge to hit 8-900 degrees bob tailing very easy, and in insite it says measured egt of almost 1400 degrees????? I don't see how its possible. Doesn't even burn the never-seize on the pyro probe? It jumps to over 600 on just the slightest throttle imput going down the road. This has been an issue sinse it was new. Has new factory gauge and pyro. Nothing I do seems to make a difference. I'm gonna try a mechanical setup. Just lookin for opinions and insite