Each ssd already comes with over-provisioning. The over-provisioning can be anywhere from 5% to 29% of overall drive capacity. The percentage varies for each brand and model. Right now certain OCZ Vertex 2 ssd's with the new 25nm NAND have the highest percentage at 29%. SSD's with the new 25nm NAND require more overprovisioning than the SSD's with 34nm MAND.
You cannot over-provision a solid state drive. It is something that is done during the manufacturing process.
Performance loss can occur for a variety of technical reasons. It could be anything from incorrect installation, incorrect BIOS & Windows settings, a firmware problem, or deteriorating ssd components.
It is a good idea not to load your ssd to full capacity. You may have files or other items that you want save or software you might want to download and save. There's always something.