The Slingbox and SlingCatcher will try and give you the best possible picture with the lowest amount of bandwidth over a remote connection. Your highest throughput will not only depend on the highest common bandwidth denominator between the two connection but the latecy between the two points. There's only two ways to improve the image quality for a remote connection. Subscribe to the service with a higher upload throughput at the Slingbox location, or reduce the latency between the the Slingbox and the SlingCatcher.
Here's a link to an post (http://placeshiftingenthusiasts.com/forum/sling-catcher-forums/slingcatcher-cannot-connect/#p850) which one user used a proxy server to improve his connectivity.