Your TV has built in scalers which do that anyway, so unless the box has better scalers than the TV, its pointless. Scart RGB is still the best way to output Sky+, or HDMI for SkyHD.
The biggest problem with external scalers, is if the TV is a slightly non standard resolution. For example a 1366x768 LCD panel. The Scart output is 720x576, then it could be scaled to 1280x720, and then the TV still has to scale it to 1366x768 to display on the panel.
For a scaler to outperform the TV, it needs to be able to output at the TV's native resolution, and have a better scaling algorithm than the TV's internal scaler.