I'm not sure I agree with that argument. At least comparing specific impulses of Proton and Soyuz vehicle engines with (older, pre-Russian engined) Delta and At

I'm not sure I agree with that argument. At least comparing specific impulses of Proton and Soyuz vehicle engines with (older, pre-Russian engined) Delta and Atlas shows the latter are often significantly higher. That's not to say the Russians haven't done good engines but RD180 for example is derived from the Energia engines which didn't have (at least publicly) lots of trials/failures.