Just a couple of small points, although I do agree with your analysis in general:Belyenko was actually a Mig 25 pilot. Read his own reports. The US deconstructed the Mig25 - they knew its capability.
First, the US did take apart Belenko's MiG-25, but they never had the opportunity to fly it. Without actually flight testing it and the radar, they had to trust Belenko's word for its capability. You cannot look at an aircraft and its parts and say what it can do with any real accuracy. I am not saying Belenko was wrong or lied about its performance, just that the fact that the US had their hands on the aircraft for a short while counts very little when it comes to the performance of its airframe or systems. What you can, however, learn very quickly when you take something like that apart, is what the state of Soviet aerospace technology was at the time.
Regardless of whether or not the MiG-25 could intercept a SR-71, it is an extremely capable high-speed interceptor. As an example, it could actually reach its maximum speed with a full missile load, since the max speed was not a thrust limit but an airframe limit. Compare this to most Western aircraft of today, where the max speed dramatically starts dropping when you add ordinance. So, regardless of whether or not the MiG-25 could successfully intercept an SR-71, or whether they used valves instead of transistors in its avionics (I note the MiG-31 is much more modern in that regard) it is still an aircraft which, especially performance wise, was an incredible feat of engineering at that time. In the context of the current thread, where people were discussing the relative qualities of Soviet/Russian aircraft and their Western contemporaries, I think the MiG-25 is a very good example of an aircraft that could outperform its western contemporaries at the time when it entered production.