The weakest link in the chain, by far, is the internet provider at your SlingBox end. The SlingBox will utilise the slow "upstream bandwidth" - which will probably be only 480k or 512k (typical for 8M downstream rates). In the UK, some of the better providers are playing around with 1.5M+ upstream, and 10M is available at certain trial sites.
In summary... SlingBox end uses slow "upstream bandwidth", SlingCatcher end uses the faster "downstream bandwidth".
Note that both your internet providers, and the "internet" cloud inbetween each end, will occassionally suffer bottlenecks - further reducing theend-to-end video stream speed.
So - the bandwidth at SlingCatcher end is almost irrelevant. You could have 1000000000M at the SlingCatcher end - but if the "upstream bandwidth" is only 512k, then that's the maximum speed at which your video stream will operate.
Best chance for a good solution is to find an internet provider at the SlingBox end with a fast upstream bandwidth.