I have a scene strip from a compositor setup, in my VSE timeline. The video clip (8bit source) that comes from that Comp node scene has an expanded color space.
In the VSE I made sure to set the Compositor's Scene strip Properties>Filter>Convert Float to active. The rendered output from the VSE scene (in the UV image editor window) is correct but the VSE preview window is very low contrast and doesn't match.

Attached is an example image of the effect. Looking at the Luma Waveform for both windows shows the effect of raised values. I cannot submit a demo .blend due to no packing function.

WinXP 32bit 2.64.9 r52376