I also just did my 2014 Pro RMK 800 and consistent with your temperature numbers. Overall decrease of 10 - 12 degrees consistently. I also noticed the cooling flow through the reservoir is much more turbulent and the system seemed to purge out any trapped air quickly.i just done this to my friends 2013 swtbck .
I plumbed in the BRP T mixing valve just like the picture and write up .
Due to limited access to fittings , I left the front cooler intact and plumbed in as is .
The system works very good , just as advertised in the opening statements of this thread .
breaking trail below 20 mph temps at 105
loose packed trail below 30 mph temps at 118
loose packed trail above 30 mph temps at 107
ambiant temps above or just at freezing ,heavy wet clammy snow
Took a brp renegade out on back to back run and it ran 25 degrees higher than the polaris , temps are extrolated due to the BRP use of bars vice numerical value
very good mod highly recommended
I also used pre mix besides the engine oil pump
The one thing I did differently, was not able to get the washer for the stock thermostat delete so I just gutted my stock thermostat and used that to retain the stock seal.