As far as I am concerned, the MAP sensor actually measures the vacuum within the intake system. If the MAP is disconnected or malfunctioning, the computer will be tricked into something that may be undesirable.
Disconnecting the vacuum line from the map sensor will add unmetered air to the intake (Unless capped off) that probably shows up at the O2 sensor as lean. Computer tries to adjust until it can't anymore. Most likely runs richer (lower MPG)
I believe it is not a good idea to take the MAP out of the loop.
As far as I know, the computer can only do a certain percentage (forgot the exact number) to close the mixture loop. Perhaps your vehicle is in an open loop at this moment and appears to be running better, but it really isn't. Open loop occurs during cold start and up to a certain engine temperature (think 40C) after that the computer needs to close the loop to run the engine efficiently.
I had some problems with my 95 E320 in this area (not the MAP) but it drove a lot better with the problem, but was extremely rich, got about 9 MPG's and didn't pass smog.

I am still learning about modern automobil's and by far not done with it.....

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Mostly, I don't know notin, I just know where to look.
I am looking back, to over 30 years in Electronics Design.
Electrons don't care if they move in a car, computer or relay!

95 W124 E320 M104.992 - Because, I love to repair, naaaah!
Over 221,000 Miles
Cheers,
Norbert