I don't know if this will help. I while back had to encode a DVCPro HD project that ran about 5 minutes. I couldn't get a file size of 5 mb's with any sort of decent quality but what did help was knocking down the audio quality to 48kbps 44.1khz or 16 bit 44.1khz or 8 bit also helps. Also I found that I could knock the frame rate down a little bit when using qt conversion from within fcp. Say from 30 frames to 15 frames. The picture quality was still very nice if ever so slightly jerky. If you find a way to get down to a file size of 5mb's with decent quality please let me know.
If it's your demo reel the first step I'd recommend is using your source material rather than an MPEG2 (already heavily compressed) source.
You're going to get much better quality (at a given data rate) if you use On2 VP6. You're not getting that with Visual Hub for sure. There's On2 Flix or Squeeze and Episode with the VP6 add on also.
Just curious, but if it's your demo and you want to sell your services/work/show the best quality, why such a file size limit? I'm usually more concerned with the data rate and appropriate frame size for it.