I am guessing the speed tests that you performed was a point to point connection from NYC to GRU and GRU to NYC individually. And NOT NYC to NYC or GRU to GRU since that wouldn't be able to test the latency or bandwidth between the two sites. As that assumed, 5 Mbps Upload speed in NYC where you have the Slingbox Pro, and 8 Mbps download speed in GRU where you have the Sling Catcher and Slingplayer are the important numbers to us. So, technically the common speed between those two numbers is 5Mbps or 5120Kbps.
Unfortunately I only get 1400 kbps or about 1.5 mb when I wach the slingtcher or the computer.
I honestly don't see a problem with that. First of all, the advertised speeds usually lists "burst" not sustained speed. The other important information is the latency between to sites since that's taken account. The Slingbox (server) and the Slingcatcher or Slingplayer (client) will always work together to give you the best possible picture with the least amount of bandwidth under your current conditions.
The Slingbox Pro is only capable of a maximum resolution of 640x480, you can verify the encoding stream statistics by pressing [Alt]+[Shift]+[i] on the Slingplayer Desktop while connected to your Slingbox. You'll be able to see the numbers which the algorythm uses to determin the encoding parameters. Something you may want to check in those statistics is the steam type should be "TCP" otherwise if its "SNATT" then you're bouncing a connection through Sling's servers.
The only thing that I could recommend to use up more bandwidth is to get a Slingbox Pro-HD. To stream in higher resolutions and while will use more bandwidth.