not really sure what ASPM is for. but I think it's not about sleep state. but more like power saving feature. google says:
PCIe ASPM technology allows PCIe hardware to transition the link to lower power states. The link is the physical interface between the PCI endpoint and the PCI switch. When ASPM is enabled, we can reduce power consumption even when the devices are in a fully powered on state (D0).
While ASPM can be enabled by the BIOS, it is often enabled only for mobile systems because the
latency to transition from lower to fully powered state can cause a drop in performance. However, depending on the administrator's power policy, the OS should be able to switch this feature on or off at will, depending on whether the administrator wants to favor power over performance or vice versa.
http://www.lesswatts.org/projects/de...ement/pcie.php
in my case, I have to set my processor power management minimum to 81% so the transition to 100% is not far.