When the J6350 first released, you had the option for PIMs (WAN type modules) and ePIMs (FE and GE type modules). The difference between the two slots are ePIM slots has PCI and PCI-X bus connection where PIM slots only PCI bus connection. The use of the PCI-X bus is the reason why ePIMs could only reside in ePIM slots.
A uPIM, however, can reside in any slot on the J6350. It had the ability to use either the PCI or the PCI-X bus depending on what slot the uPIM is installed in. Naturally you can expect better performance overall with ePIM slots. But also keep in mind overall maximum performance capability of the system as a whole. You can get that info from the datasheets.
Hope this helps.
-Richard