Interesting little (non scientific) comparison. I have (amongst several others) a lightweight 29er XC bike and a Stumpjumper Expert 6fattie.
I have the same XT brakes (the M8000) on both ends of the XC bike, and on the back of the stumpy, with a Saint 4 pot on the front of the stumpy. All are using Shimano Ice-Tech finned metallic pads.
On the stumpy I have the Shimano Ice-Tech rotors, 180 front, 160 rear, on the XC bike I have some older Hope Floating Saw Tooth rotors, also 180 front, 160 rear.
All 4 brakes have been bled recently, lever feel is good and in no way spongy, but they just seem to have much more bite on the XC bike, less (like, quite a bit less) lever effort will quickly lock either wheel if required. The wheelset/tyres will be a lot heavier on the plus bike, and it will have more grip when stopping (in most circumstances) but rotating mass and grip aside, it feels like the Hope rotors provide much more friction than the Ice-Techs.
Both bikes stop perfectly well, is this likely to simply be the big wibbly 3" tyres and sturdy rims trying their hardest to keep the stumpy rolling, or could the Hope discs actually be a more effective steel alloy?
They may well offer slightly more friction by way of great surface area, smaller holes or whatever - but my money would be on the lighter built XC wheels and tyres making them easier to slow and of course lighter, less grippy XC tyres will lock an awful lot easier than plus tyres!
Anecdotally I've had the same. I usually use Avid G3 or whatever and compared to some TRP and Promax jobbies that came with bikes, they are betterer in the stopping department.
Wheel size/tyre/weight makes a huge difference to braking. I first realised this with my moonlander, difficult to gear brakes powerful enough to stop that big wheel once it got going
Skidding isn't stopping....
Have you done a test to see how actual stopping distances vary?
or could the Hope discs actually be a more effective steel alloy?
My guess would be the huge amount of sharp edges on the hope 😆
Pay off is varying performance in different situations which the shimano rotors will be more consistent but obviously lack that bite. Hope will probably eat through organic pads a lot quicker too obviously.
Swap the rotors over, see if performance follows
Only disk I've ever had fail on me in use was a Hope. Never bought another so can't back that up with more data though.
I used to run weightweenie KCNC rotors....until the brakes faded so badly I had to do an emergency laying-down of bike (and self!) approaching a tight corner at the bottom of a road decent (in Devon, it's not like I was cooking them down an Alp).
The LBS owner I was riding with was pissing himself. When he recovered he convinced me to buy Shimano.
Xt have less modulation than the saints for starters.
You can only compare the rotors if the bikes are identical.
Only time I've noticed a difference with discs is on my gravel bike when I went from shimano 'organic only' discs to centre lock xt when I bought some Hunt wheels.
Same rotor diameter same sintered pads- far better performance from the xt discs.
Only difference was that the xt were surface milled/ground whereas the organic discs wernt.