You can use relatively high bitrates on Bluray since you're not hurting for space.
The bitrate I have given are for 2.0, so AC3 is instantly 100% ABXable at bitrates that are considered high/to overkill for any other modern lossy codec.
The idea that high bitrate would help the quality of AC3 to be better is IMHO a myth.
Castanet is not the worst sample around anymore, Harlem & Autechre are worst, but AC3 Soft Encode which, back when I tested it, was supposed to be one one the best AC3 encoder around (maybe SurCode or Sonic Foundry are better but I got a nasty virus before I could try those... I had to re-install XP despite antivir), is already not transparent at 256Kbps on castanets.
If you consider that a killer sample is only 20 sec, but that a movie lenght is 1h30 to 2hours, this leads to the simple conclusion that the AC3 stream from blu-ray is likely not transparent, & it's worst for DVD as the bitrate is lower. AC3 is not robust enough to stand the shock, it's like encoding a CD at 128kbps MP3 & hoping that it will be 100% transparent. It might be only if you don't listen carefully.
One conclusion that you can draw for almost certain is that any movie soundtrack with applauds in AC3 (which means any live music) is not transparent no matter the bitrate. The sellers are not idiot, that's the reason why most live music DVD comes with a 1536Kbps DTS stream.
Here is an equivalency table between AC3/DTS 5.1 bitrate used in physical media & 2.0 bitrate used for music: (with my non-100%-scientific opinion next to it, but the extrapolation is still based on some ABXing)
DTS ------ 5.1 1536Kbps /5x2= 2.0 614Kbps Good, most likely near transparent.
DTS ------ 5.1 0768Kbps /5x2= 2.0 307Kbps Not Good, not transparent, most likely easyly ABXable.
AC3 BD --- 5.1 0640Kbps /5x2= 2.0 256Kbps Not Good, not transparent, most likely easyly ABXable.
AC3 HD-DVD 5.1 0504Kbps /5x2= 2.0 201Kbps Bad, not transparent, instantly ABXable.
AC3 DVD -- 5.1 0448Kbps /5x2= 2.0 180Kbps Bad, not transparent, instantly ABXable.
It gives you a better idea of how to compare 5.1 bitrate to 2.0 bitrate & will help you realize that high bitrate doesn't help AC3 much.
The fact that AC3 is already used at high bitrate by default is an hint of how awfull it must be at low bitrate. Overall AC3 doesn't even compete with a bad mp3 encoder.
... but there is one hope, I heard on doom9 that professionnal AC3 encoder were better than those available to mortal users like us ... (I also heard that hope is what dies last ...)