The SLA5500 is a UPnP device, but unfortunately not compatible with the RC9800i. Media Center PC extenders will generaly not work together with RC9800i over WiFi, since they do not use the UPnP protocol to communicate with other devices. The infrared part will work.
At the moment the supported UPnP Media Adapters for RC9800i are the SL300i and the SL400i from Philips.

Regards,
Philips Support RC9800i