I bought a Slingbox 350 today, and a Chromecast dongle - specifically because I'd read about Slingbox supporting Chromecast on the iPad app.
I can transmit the tv signal onto a different TV, that is fine - but the aspect ratio of the displayed picture is wrong. I have thick black bars on the left and right of the picture, and thinner bars top and bottom. The displayed image is "squashed" so people look very thin.
I can't see any settings to change that make any difference.