If it was just a CPU swap into the Nvidia chipset, it would happen as soon as the CPUs met Apple's price/system specs.
Since there is a new chipset involved, that will take a bit longer.
However since the iMac is leading this time around, who knows. 😕
Gots to wait for an i5/7 teardown to see which chips are used, it they are the desktop versions ... that would be a case of monkey fur caught in the wrench.
If they are mobile qualified parts, expect the MBP 17 to get it soon with a outside chance at the 15.