Well, there is no doubt that even when the 5K Mac is maxed out, it is not the most powerful machine there is.
A more powerful machine is a workstation class desktop or a top end gaming computer.
Luckily I am merely a consumer user so even the quad core i5 on my 5K iMac is powerful for my needs.
But what none of those little PC workstations have and the reason I still recommend people to go Mac (maybe Mac Pro if you need that level of performance), is that Mac has OS X. OS X or macOS is so superior to 'garbage-ows' (Windows), that it is difficult to even put an exact dollar amount on how much better it really is.
While I would not classify the iMac as the most powerful machine available because of where it lies in the objective technical hierarchy, for a consumer level machine, it flies, and that is what I'm concerned about.
For graphics professionals I recommend the Mac Pro with its dual FirePro GPUs. For someone like me who doesn't use it for that, the iMac is undoubtedly the best there is.