As a software engineer, a firmware change log can immensely interesting to geek out on/analyze. I think the support guy admitting to mistakes (changes) to voltage accuracy is impressive. He is saying that all firmware out there for the given product is inaccurate w/out that update. He didn't quantify the inaccuracy because slight is relative. I'd be surprised if they supply an accurate change log to the public.
As you mentioned, there is risk of breakage. On one hand "if it works, don't fix it", but you could get more accurate voltage readings with the update. You could ask if the FW is downgradeable in case you have a problem.