Is the chainline completely out of whack with this setup?
A Shimano M8000 double has a 48.8 mm chainline which as a single would work out as a equivelant 51.8 mm single chainline.
Any OPEX?
Can get one considerably cheaper than single to background question

The M8000 double crank is the same as the M8000 single crank. There’s no difference in the chainline.

Ive never had any issues with it, just mount it in the middle ring spot and bobs yer uncle

scotroutes, that was my understanding until I read OneUp components guide to 1X

I run a single ring on numerous double cranks and I’ve never noticed any detriment to shifting.

I’m running an slx double in a 1x setup on a boost 148 frame, I think that gets it within 1 or 2mm of where a boost specific 1x crank would be.

I find it better, although in theory the chainline is a couple of mm outside dead central whereas a triple in the middle position would be spot on, with an 11-40 10 speed cassette most of the gears i ride are to the outer part of the cassette (say 5-6-7-8) so the chainline is better for them.

It does mean that the low gears on the cassette are a little cross chained but they are climbing gears and bail out mainly anyway. Might be different if your riding is hillier.

If you buy a 1x Shimano M8000 crank and install a ring in the “big” position it’ll be about 52mm. You can install a small spacer/shim to move it in a little but, in practice, it seems to make no difference.

FWIW I run a 2x non-Boost M8000 crankset on a Boost frame and it’s fine.

I also run a 1x non-Boost M8000 crankset on a non-Boost frame – and it’s also fine.