In the datecode the first digit is the final digit of the production year, while the letter that follows it stands for the production quarter of that year. So the critical datecode 8C stands for the third quarter of 2008, which is the period when the new bios that blocks FMCB was introduced in the production of new SCPH-900xx consoles, which before that used bios v2.20, working fine with FMCB.
This use of the final digit of the year means that not only datecode values 'larger' than 8C can be newer than that period, but also some with a much 'lower' numerical value, which never the less represents a later date, since (for example) 2nd quarter of 2010 would have a datecode of '0B'.
For recently bought (meaning this year) new consoles there is usually no doubt though, as all of those probably have the new bios making FMCB use impossible (unless some store had overstocked badly on old consoles, which is unlikely).
Note also that for consoles with lower product code than SCPH-90000 there is never any need to check datecode, since they were made long before bios v2.30 was created. All of those older models are compatible to FMCB.
Best regards: dlanor