This may be true for Vietnam, where indeed the USSR got their hands on quite a few modern US equipment, though only bits and bobs (AIM-7, AIM-9 variants which resulted in K-25 and R-13M, radar and other electronics bits etc.), but still not the same as having say a working F-4 to test imo. Besides after Vietnam as i understand there wasn't much modern US stuff USSR got their hands on, at least not that we know of.
So what informed folks think, would it be worthwhile to copy the AWG-9 if they get an iranian F-14 early enough? How about if we make it so either Iran buy F-15s as well (apparently there were discussions) before 1979, so USSR gets an intact F-15 too, or we make such that the israelis lose couple of F-15s and F-16s in 1982 but conveniently the wreckages are recovered by the syrians/soviets (there were certainly some close calls for F-15/16 losses even according to IAF themselves) would the soviets be better off copying the APG-63 and APG-66 in the 1980s, or at the very least use them to improve the N-001 and N-019 as much as possible?
How about copying the AWG-10, it seems from the data recently discussed here it had a quite a bit better range than even S-23ML!
Or things like an american Tolkachev?
Hell, what about all the american stuff mentioned above somehow ending up in China as well?! Imagine chinese F-14s flying alongside chinese MiG-23s (i know they haven't copied the MiG-23, just bits of it put on J-8-II). Though probably this might merit a different topic.