Seatpost shim 31.6 to 27.2 good or bad idea

Yes, used for years on an MTB, no issues whatsover

+1 to the above. I have a USE shim to fit a nice & springy KCNC seatpost (30.9mm) that I got half price, into my Condor (31.6mm) & it's been flawless.

Again, I used a USE thermoplastic shim on a my old Whyte 19 hardtail. Once i got the setup dialled, no problems at all and it had the effect I suspect your looking for.

I use a USE shim to fit an Ergon 27.2 seatpost to my 31.6 seat tubed boardman. works perfectly. No creaks, no slips. Helps build in a little more comfort

USE thermoplastics all round here, they are machined to avoid slippage, lather them in plastic paste and jobs a good one.

USE shim and never had a problem with creaking ir slipping.

I use a cane creek one. The seatpost slipped a bit for a while (carbon into alu shim) but has stabilised now.

I use one that came with a TT seat post, works fine no creaks, fitted on carbon bike