Extension cable for PCIe

Internal PCIe interfaces are using a common Refclk architecture, as far as I understand. By specification, the data/clock transport delay delta t1 must not exceed 12 ns (see PCIe Base-Specification V2.0, 4.3.7.2.2. Common Refclk Architecture). Considering various on chip and motherboard wiring delays, a 1 m extension cable can't keep it. I don't know however, if the parameter is critical. Apart from this problem, I'm only aware of short (0.1 - 0.2 m extension cables) available as retail products.