AnandTech investigates the relationship between spare area and performance consistency in today's solid state disks, you can read the article over here. The conclusion is that minimum performance improves tremendously once you reserve about 25 percent of the disk as spare area, although the effect is smaller on disks that have a controller that prioritizes IO consistency, like the Link-A-Media based Corsair Neutron.
For drives on the market today that don't already prioritize consistent IO, it is possible to deliver significant improvements in IO consistency through an increase in spare area. OCZ's Vector and Samsung's SSD 840 Pro both deliver much better IO consistency if you simply set aside 25% of the total NAND capacity as spare area. Consumer SSD prices are finally low enough where we're no longer forced to buy the minimum capacity for our needs. Whatever drive you end up buying, plan on using only about 75% of its capacity if you want a good balance between performance consistency and capacity.