slingbox performance on the WAN, so outside the home and local area network LAN is really most impacted by outbound internet speed streaming capability. Have you run the test at speakeasy.net/speedtest ? Your OUTBOUND speed will tell you what you will get on the WAN.

With a decent 250-300kps stream, you'll get a pretty decent stream anywhere. With a nice 450-650kps stream, you'll get a nice step up in quality, actually more than quite decent.

With 700kps+ you can get HD quality streaming, but would need an HD box for that.