The speed rating is not how fast the chip runs. It is a 'maximum that chip is to be run at'. Even if you put a 10 MHz rated 8087 into your 5150/5155/5160, the motherboard will still only run the chip at 4.77 MHz.
Some people are under the impression that adding an 8087 to their 5150/5155/5160 motherboard will speed up their computer.
That impression is false.
The 8087 does not speed up hardware. It will speed up software that is written to take advantage of the presence of an 8087.
The following threads at the Vintage Computer Forums discuss such software:
Thread #1 ("Software which can use 8087/80287 coprocessor?") Thread #2 ("Programs that use the 8087") Thread #3 ("Programs that use the 80287")