Given that the speed you'd be convicted of (8 km/h over) is such a meagre offence (really? 8 km/h?! Who tickets for 8 over?!) I'd go to court and see what is in a plea bargain, if nothing else.
Fight the ticket, and put in a disclosure request. The disclosure request will help you get the evidence the Prosecutor intends to use against you. For speeding offences, this usually means you'd ask for the officer's notes, and relevant parts of the laser/radar manual. Those are what you need. Take a look at the "Courts and Procedures" section of this website for advice on putting together a disclosure request. Once you've got the disclosure, you can see if there's anything that's missing or incorrect.
Otherwise, if your trial date is more than 11 months from the date of the offence, you can also file an 11b to get the charges thrown out. (See "Courts and Procedures" section.)
There is no requirement in the Highway Traffic Act or in the Ontario Traffic Manual that requires "Reduced Speed Ahead" to be posted. Some municipalities may have by-laws requiring those signs to be posted, or they may do so for safety or awareness reasons, but it is not required provincially so the absence of such a sign won't help you. Likewise, there's no law requiring any sort of "education period" where you can get pulled over and simply warned "hey, we changed the speed limit sign." Sometimes, as a courtesy, they'll put up those big circular "NEW" signs, but whether they are present or not won't help.