It was seen by Soviet pilots and radar (it was poorly visible but not invisible). As to top speed, from what I've heard that's a matter of how much wear and tear you're willing to incur.
So in theory, could be Mach 4-6 already in an SR-71?
If the SF71 was like many other military aircraft, then it could be flown far beyond the long-life envelope. Overclocked, if you will, getting extreme performance for an hour at a cost of years' wear on the engine and body. You can see how this approach might make sense for an SR-71 where one might sensibly want to sacrifice the aircraft to get some pictures out. You can also see how this complicates defining "top speed".