Originally Posted by Nagorak
If it's the same as Gear Down Mode on Ryzen then it shouldn't have more than 0.5% to 1% impact on performance, since it allows you to tighten the other timings more to compensate while remaining stable.
In terms of achievable performance (after manual memory tweaking) I don't think that should result in a big performance difference, but it will probably look bigger in reviews since the reviewers will just run XMP and won't tighten any of the other timings to compensate. (To be fair that will reflect the average buyer's experience too.)
The performance difference probably isn't huge, it just seems a bit misleading by intel to claim 3200mhz support on the i7 only for them to limit it to gear 1:2 mode. Looks like artificial segmentation to give the i9 a slight boost over i7 in benchmarks.