Chosen Solution
A 2015 2.8ghz 16gb mbp logic board is $381. Would this work in a 2012 MBP retina body? swapping the logic board to get the faster processor?
You could look into a site such as this http://www.powerbookmedic.com/ and enter information for both model numbers (and build year) and look to see what their part numbers are for each. If there is no shared replacement for both, and if the physical ports and other hardware have been changed (to not fit the case, etc) then the answer would likely be No. Usually as the hardware configuration changes, so will the custom aspects of the logic board for the build series; just because the sales model number or other number is the same, does not mean anything inside the case would interchange.
Sorry its just not possible! The 2012 retina is very different than your 2015 board. It won’t fit or work.
