1. For a two hour movie using the superbit approach.
2. Using CBR LPCM 5.1 24/48 for easier calculations. DTS-MA/TruHD can improve HD DVD video rates.
3. BD video rates are maxed regardless. HD DVD video rates cannot be maxed regardless.

LPCM ~ CBR ＠6.9 Mbps = 6.21GB
ACV/VC-1 ~ CBR @40Mbps = 35.16 GB (BD) / CBR @23.34Mbps = 20.52GB (HD DVD)
HD DVD w/ TruHD ~ ABR @3.4 Mbps = 3.06GB / Video ~ CBR @26.84 = 23.59GB

BD50 = 41.37GB ~ 8.63GB available for extras
HD DVD30 = 26.73GB ~ 3.27GB available for extras
HD DVD30 (w/TruHD) = 26.65GB ~ 3.35GB available for extras

Even with the superbit approach there seems to be quite a bit left for extras. Please check my math for errors.