after fully read all how to install the 10.12 in cMP 3.1, it seem not easy at this moment, in past I'm try to looking some second hand 4.1 and flash to 5,1, but I'm very afraid this 5,1 will not support the next 10.13 or 11.
Just my 2c. It seems Apple pick a date (may be the expected 10.12 release day -7 year) and draw a line at there to define which Mac is supported in 10.12. Otherwise, there is no reason the weaker 09 LATE iMac and 09 LATE Macbook can install 10.12, but the stronger 09 EARLY Mac Pro, and 09 MID Macbook Pro cannot.
They didn't have any pattern on software support in the past, but to me, this time, they seems want to follow the same 7 years rule as the hardware does.
If that's the case. The Mac Pro 2012 (5,1) should be still supported on the next 2 OSX update. Of course, if follow the same pattern, they should drop the 2010 Mac Pro on next OS upgrade as well. However, that has to be done very artificially, because both of them identify as 5,1.
This year is very clear, both 3,1 and 4,1 are out. But 4,1 user can flash to 5,1 and stay inside the circle. The real interest part is how they handle it next year. Will they support the 2010 Mac Pro, or drop the 2012 Mac Pro?