Can I upgrade my iMac 27" Core i7 (5K, Late 2015) Graphics Processor?

Can I upgrade my iMac 27" Core i7 (5K, Late 2015) Graphics Processor? 

My current iMac 27" Core i7 4.0 5K, Late 2015 machine has an AMD Radeon R9 M395X graphics processor with 4 GB of GDDR5 memory.  The spec's on this graphics processor indicate that the bus interface is listed as "MXM-B (3.0)”, and its memory bus is “256 bit”, with a TDP rated at 250 watts maximum. 

 So, Choice # 1:  Can I swap this out for a AMD Radeon Pro 580X with 8 GB of GDDR5 memory?  The main difference is that this processor has a PCIe 3.0 x 16 bus interface.  Its memory bus is the same “256 bit”, and its TDP is rated at only 150 watts (would this mean that there wouldn't be overheating issues, because of the lower power consumption?)

Choice #2:  What I'd REALLY like to do is put in an AMD Radeon Pro Vega 64 graphics processor with 16 GB of HBM2 memory - this also has the PCIe 3.0 x 16 bus interface, but it also has a different memory bus, listed as “2048 bits”, and a TDP rated back at 250 watts.  

Bottom line question - does the bus interface (PCIe 3.0 x 16) in both choices STOP me from upgrading?   And, if not, does the memory bus difference (2048 bits) in the second choice STOP me from upgrading?

Any knowledgable advice would be most appreciated!