Something I've just noticed, if you're encoding to 480p you may hit compatibility problems. I think that both AVCHD and Blu-Ray specs only allow for interlaced content at SD resolutions.

If you're keeping quality at a maximum, why not keep the audio as uncompressed PCM instead of encoding it to Dolby Digital? You should still be able to get a bitrate around 7Mbps with the 8GB limit of DVD9 if you really want to. Even 8Mbps seems quite high for 480i when encoding to h264/AVC, so you should easily manage the quality levels you're hoping for. ;)