I have a large collection of 25 mhz 286 cpus and have benchmarked them all and recorded their fastest stable benchmark speeds. Please note that this involves running a benchmark of a few minutes - in general you will have to take 1-3 mhz off that speed to get their stable speed, and i would recommend a heatsink and/or air cooling if you are serious about running the chip to last a long time. These benchmarks ran on a VLSI SCAMP-based chipset. It's possible other chipsets may run tolerate running these chips faster or slower, but their speeds should in general still correspond linearly with my recorded results.
I'm selling off a lot of my extra chips (not the very fastest). I have parted these into ten sets of 5, more or less all equivalent in total overall speeds. In general you are receiving in each set:
Two NOS Intersils chips that benchmark ok (27-28 MhZ max speed)
One Slower used Harris chip that benchmarks low (22-25 MhZ max speed)
One Medium used Harris chip that benchmarks ok (25-28 MhZ max speed)
One Faster used Harris chip that benchmarks well (28-31 MhZ max speed)
The Harris chips are all used and have comparatively bent pins compared to the Intersils, but they do all work as I have weeded out any nonworking chips or fakes. The Intersils in should handle their rated 25 mhz speed fine, and are in good condition.
Please note that in general, you will be limited by RAM speed and not CPU speed on a faster 286. If your machine is not working at these speeds you may need to set DRAM wait states in the BIOS.