PCIe 2.0 and PCIe 2.1 are essentially the same thing,and there's no differences in speed. Supposedly, PCIe 2.1 adds for improved compatibly to the forthcoming PCIe 3.0.
Whatever this statement means "PCI Express 2.1 supports a large proportion of the management, support, and troubleshooting systems planned for full implementation in PCI Express 3.0." and have fun reading the specs -> http://www.pcisig.com/specifications/pciexpress/base2/#b21 {members}.