Interrupts: Line-based or MS(I)? Your opinion and experience?

I KNOW IT'S VERY LIKELY PLACEBO EFFECT, but onscreen mouse movement really does feel smoother since switching the USB controllers to MSI. Everything else, including GPU, not so much.

Has anyone else wasted their time doing this, and did you notice a difference?

It is not placebo. MSI and MSI-X are a much better and faster interrupt mechanism. However, I would not manually configure the interrupt. I would always let the driver configure it. It will vary from core logic and driver; and can be very pissy when it is not enabled in the driver with Windows but Linux will enable it. Pisses me off even more that nVidia does not have MSI enabled on Windows desktops, but on laptops I have seen them enabled. They most likely are with workstation cards-I am pretty sure they would have to be due to SR-IOV.

I enabled it on my NVidia gfx card many years ago when there were glitches in framerate.
It did help reduce them a lot.
I still enable it again after each driver install but havent checked whether it does anything these days.
I understand that AMD gfx cards automatically have MSI mode enabled.