Shims on gravel bikes - would it work?

I’m about to order a Kona Rove NRB. I like almost everything about the bike, but the 31.6 mm seat post I’m a bit worried about. Wouldn’t that turn out a bit stiff? Have anyone user shims on a gravel bike? I understand it should be avoided on some carbon frames, but in this case we are talking about an alloy frame. The shims I found does look kind of short, just 80 mm.

Don't do it! 

I’ve used shims in mountain bikes with no issues. As long as the shim extends a minimum of 1 inch below the bottom of the top tube/seat tube junction it should be okay. But you probably should check with Kona. It may also void the warranty.

An appropriately design shim should be fine. I use a 31.6 to 27.2 USE delrin on my mountain bike with no problem. 

The seat tube on the Rive NRB is prolonged quite a bit over the top tube. So a 80 mm shim may not pass the top tube all that much I am afraid.

Missed the picture in my comment above...

The seat tube does extend a bit.  The Cane Creek shims I have are about 98mm, I don't know if you will find any longer than that.

That's fine. The shim should be at least as long as the 'minimum insertion' of the seatpost. The minimum insertion is usually 80-90mm off the top of my head. I think as long as you have at least an 80mm shim, it should be good.