One side effect of trying to install SteamOS is realizing doing the base install & updates while hardwired is a pretty good idea. However, I did take the advice of JayZ and disconnect the PC from the network for the initial install to avoid having the Admin user tied to an email address. I've also created individual normal users for each member of the family.
The actual Windows 10 install & update went smoothly.
Then came the big test - I installed Steam, CS:GO and did the timedemo. Unfortunately with the default "high" settings I didn't hit my desired [email protected] target (although I did get over 120Hz, and significantly better than the sub-60Hz I got on the iMac at low settings. The bottleneck appears to be the graphics card as it's pegged to 100% and none of the CPU cores are.
On the one hand I'm disappointed with the result - although it's not like I had any real reason to expect it would meet my target out of the box. Rather than throwing money at the problem and spending hundreds more to get higher performance I went with the CPU & graphics card with the "best bang for the buck" - which means both cheaper and lower performance. But I am kicking myself for not going with the GeForce GTX 1660 Super.
But I've done some more testing and I can get over 170Hz by dropping the quality settings, so now I just need to decide which is more important - pretty or fps, and see which settings get me the most fps for the least visual impact.
And it's not like it's difficult to upgrade the graphics card if I decide in 6 months I really want to spend the $$.
Now begins the long process of setting up the various programs I want to use on the new PC, configuring them to work the way I want, and consolidating the files from the iMac and my work laptop.