If you're getting 1.3mb speeds when connecting to your slingbox remotely, then trusted proxies may help you in getting better speeds and a more stable connection.
However, there are two points you should be aware of:
1. If your upload speed is slow where you slingbox is located, then trusted proxies wont increase your speed.
2. The new watch.slingbox.com does not support proxies. Neither do any of the mobile apps or connected devices (like WD TV Live).
Everyone is still waiting for slingplayer devs to put this feature back to the watch.slingbox.com.
However, if you are fine with using the old desktop application, it can support proxies. (Although slingbox reps no longer support the desktop app.)