I suppose I wasn't very clear: Yes, I know that Arctic Silver itself comes in a syringe.

I have a tub full of it (as in: a tub full of thermal paste and other computer-fixing-related objects). You wouldn't believe how much I've had to use it over the years. Surprised we haven't seen Arctic Silver 6, really, but AS5 is about as good as it gets. Not used GC extreme but I've heard good things.
Anyway, back on topic:
I think the M295X is going to throttle no matter how much thermal paste you put on there, but I'd LOVE to be proven wrong, of course. Until then, I have no intention of ripping apart my iMac. I have numerous adhesive-strip kits to put back the 2012/2013/2014 screens on, so that's not a concern.
And I love my 5K iMac. There's no time like the present, and while it does run hotter (yet to be determined to be an issue), that screen is just the greatest thing since bagels and cream cheese.