Having put XT and SLX 12sp on mine and the wife’s bike in the last 6 months I have to say I’m impressed with the shimano rings. Cranks are nice too and just work, I was looking at some raceface ones purely from a cosmetic point of view but decided that the extra cost just wasn’t worth it. The Shimano ones work brilliantly, are the easiest cranks I know of to install and I can’t see them while riding or after riding due to mud so IDGAS!
I wasn’t sold on the chainrings at first as they look like adapted 2x/3x cranks but the teeth are steel and if reports are anything to go by outlast alu chainrings by quite some margin. I cleaned mine for the first time at the weekend and the teeth still look brand new (Strava says that I have done just over 1300km on it). The tooth ring is the same on SLX and XT, but the SLX is a painted spider while XT is anodized, so you can buy the cheaper ring (£28 for SLX 30t ring I think, so much cheaper than many aftermarket alu rings anyway) for XT cranks (although I think the SLX cranks look nicer).
Neither of us has lost a chain either, without running a guide of any sort, or had any shifting problems. Admittedly we aren’t EWS racers but we did spend a week at the Golfie/Inners without issue and regularly ride our local “enduro” trails around Dundee/Dunkeld/Perth.
If you are buying a groupset I wouldn’t worry about performance from the stock set ups unless you really wanted an oval ring. Anything else would be purely for cosmetic reasons, I don’t think any aftermarket ring/crank combo would deliver appreciably better performance (probably actually worse, particularly after some wear on alu chainrings) than stock Shimano when used with the rest of the drivetrain.
