I know someone who filed for B2 visa extension. The form was sent to the USCIS(in June) about two months before I 94 expiration date(August). USCIS sent the receipt right away, but did not make a decision until about four months after(December) the original I-94 expiration date(August).
While the application was pending the person was in the US. So in December, they sent the new I 94, which made her eligible to stay in the US until February.
The processing time that is shown on the USCIS webpage is not so accurate, and hence cannot really be relied on. The decision came way before the estimated date according to the website.
But to my knowledge one can stay in US even after your I 94 has expired, if you file for the extension about 45 days before I 94 expiration date, and it is being processed. There will not be any problems later if the extension is approved. However, they might only approve three months of stay the next time you visit.
If it is denied then the person has to leave the country within a short period of time(10 or 30 days not sure) and the multiple entry visa will be void.