The Americans clocked it at something like Mach 3.2 across the Sinai desert if I recall the incident correctly. When some Russian chap defected to the West he took a brand new MIG-25 to do it in. The Americans stripped the thing to bits and sent it back to Russia in component form.
They questioned the bloke (I can't remember his name - Pavel/Dmitri/Yuri or something) and he said the engines could power the plane to exceed Mach 2.8 but they would be destroyed in the process. They weren't supposed to exceed Mach 2.5 because the throttle control would become unstable that high it could jump about between Mach 2.5 and Mach 3 with the pilot having little say in the matter, destroying the engines in the process.
We're far too conservative to build something like that
And yes, MIG and Sukhoi are both Russian. Are the government still considering merging them?