If I fully load 8GB memory card onto my Nokia 5233, then is it possible that my phone gets slower because Nokia says it is supports up to 16GB?
The data that ur phone will read will depend on the read/write speed of the card, 8gb or 1 gb doesent matter the speed of the card matters!
The problem will be when trying to load pictures, video, msic or any other file stored on your card the phone has to list all of the items in that directory and considering that it's a phone and not a computer it will be slowed down.
Your phone has a 434 MHz processor, it's no super computer so compiling a list of several hundreds of files will slow it down.
It is not recommended to fully load memory card always live a small almount like 5 to 10% of empty space in order to have the required temp files space. If the memory card has a good read/write speed then no problem.