No, in fact I'm figuring bitrates right now to burn DL roughcut disks for proofing.

Depends on your eventual need to package and deliver. Dual-layer media is a bit more expensive but not much when doing high volume duplications. You'll make up the cost most likely in packaging and fullfillment cost savings. If the project can be done on one disc, I'm all for just using one disc. Posting a 12 hour seminar right now where each disc has to be it's own. Would just love to have 3 discs in the package instead :)

Here's an old post about various ideas about how to cram lots of stuff on one DVD:

6 Hours on a SL DVD - why stop at 6?

Once you are willing to accept compromises in quality, you can cram an amazing amount of video onto one disc. Probably the longest I've seen on commercial discs is the original Saturday Night Live discs from the first season. There are three episodes per disc. Those shows were 90 minutes long, but probably about 75 minutes without commercials. That's 3:45 stuffed onto one disc. And, it was 29.97 video, not film, which doesn't compress as well. Very impressive stuff.

For a 7.95GB DL disk, a 240 minute DVD works out to a total average bitrate of 4.5Mb/sec. Take away .2Mb/sec for audio, and that leaves you with 4.3Mb/sec for the video average.

I've done some bitrates in that range using CinemaCraft, which is an excellent VBR MPEG2 encoder and has the ability to do multiple passes. I've never seen any advantage going beyond 4 passes, though. I have no idea how MainConcept will handle that low a bitrate. If this is largely a talking head video, it should look great, regardless of the encoder.

If you're really pushing the bitrate down even further, one option is to encode at half resolution. I've never done it, but understand that it looks surprisingly decent, and certainly better at very low bitrates than the macroblocky results you'd get from full-rez encoding.