Major Yes they did accelerate to evade SAMs. They couldnt however out run them. The state of the art missiles back then could reach Mach 4 while the SR-71 had a top speed of 3.5 or so. What really occured is the guidance computers could not adjust to the sudden increase of speed. At that alt the SR-71 could still gain speed in a matter of seconds throwing off the intercept point. I have read that with the SR-71's speed SAM sites had only 30 seconds of a launch commit window. That is to say only 30 seconds of time to track, identify, and launch before the missiles would not be able to intercept before reaching maximum range.
JE you have peaked my curosity. Will you elaborate or post links? I have spent hours searching for technical reports on this aircraft. I know that they are designed to leak at ambient temperature. Once flight temp is reached however they expand and the joints sealed.