After changing my G37 manual transmission clutch recently, I noticed the power and the fuel miles per gallon (mpg) seemed to drop about 15% - it was not an issue with clutch slipping or adjustment. It was not a transmission issue.

In many of the forums and websites, it is not mentioned that the tilting back of the engine when removing the transmission can damage the intake hoses. The factory service manual (step 13 of removal -12/2011- version) mentions "NOTE: By placing wooden block between oil pan (upper) and front suspension member, the removal of transmission assembly from engine becomes easier". They do not give any further details, directions or diagram with the wood in place. I initially did not do this, but then went back when trying to get the transmission off. I am not sure about this making it easier, but they do not clarify why you should do this.
Upon completion I observed the power and fuel economy were terrible, but the transmission and clutch were working as expected. I eventually began getting errors for the manifold Absolute Pressure (MAP) sensor failing, Most research stated it needed to be cleaned or a software update applied, but at its root, the error was saying that the sensor was reporting a drop in intake volume, After much review (and cleaning the MAP sensor), I remembered that one forum (or website) that I reviewed for the clutch removal had mentioned to disconnect these hoses prior to lowering the transmission.

I now see that the hoses will tear because the transmission leaning back will pull on the dual air intake hoses and split the hoses where they meet the throttle bodies. You may not see it with a quick review, but the hoses will split on the accordion style folds and the engine will suck in unfiltered air at that point without the air filters. This air intake leak will be past the Mass Air Flow sensors and will screw up the vacuum and air/fuel calculations.

I replaced both hoses and that appears to have corrected the issues. I will need to clean the throttle bodies due to the dirty air from driving about 800-1000 miles before the issue and solution was identified.

 

In any car, when you remove the transmission, the engine needs to be held in the correct position.
If hoses are being "torn" you have allowed too much movement.
That's why the FSM says to add/slide in a block of wood to hold the engine in position.

I know this for a fact, as I just changed my CSC in my garage with the car on jack stands and worked on my back.
Whenever you remove anything, you need to think about what support is needed.