Yea I mean 0.25" is very much within the margin of error for estimating any given length without actually having a ruler on it (sometimes up to 0.5"), and even then, it could easily fluctuate that much based on the day. That being said, it would definitely be "noticeable" if two guys were in the same scene together and one was 0.25" longer.
Mandingo was likely 9" in his prime, but he could have been a fraction less because it was definitely never "proven". Danny D was almost certainly never 8.5. While I suppose it's "possible", I think he was 8-8.25. Lex was definitely never 8.4 (?) I personally am quite certain he was around 8 in his prime, but no chance in Hell he was over 8.25.