The issue with GPU support is a glitch with Lightroom itself where some of the changes are not rendered in the main window until you make another change (even though the changes are rendered in the preview). So for instance, if you have GPU support on, and you convert a negative, it may appear that the negative has not been converted. But then when you click something inside the Negative Lab Pro window, the render in the main Lightroom window will update to show show the conversion. It can be really annoying, because it is always one step behind.
The good news is that this behavior will be fixed with the release of Lightroom Classic 9.0. I’ve been working with Adobe to document the issue and get it solved. I have a version on my system that is working as expected with GPU support on.
The speed improvements will definitely vary from system to system. You can of course test Lightroom with the various GPU settings to see if it speeds up your system or not. On my 2015 MacBook Pro, I do see a small improvement. On my wife’s MacBook air, I actually see a bit more lag with GPU on. I would expect to see a bigger improvement with your new iMac. In any case, it’s worth testing and seeing how it works with your setup!