"Best Quality" for Blu-ray in Studio 12 is 25mbps, using the MPEG format option. However, the average bitrate of the video may in fact be lower, depending on motion content and the native specs of the camera used.
25mbps is probably the max bitrate for tape-based HDV MPEG2 videocams. Some Canon AVCHD models shoot at 24mbps. Most others max out at 17mbps. The Panasonic SD3 shot 1920x1080i AVCHD at 13mbps.
Bitrate in video can be as misleading as pixels in still photography. More might be better, but not necessarily, and sometimes more simply means consumption of more memory space and a bigger PITA when editing.
The Canon EOS 5D Mii shoots h.264 1920x1080 30p at up to 34mbps. That camera has a large sensor, perhaps adequate to generate a real HQ bitrate of that sort. However, it would be superfluous for some scenes, and Studio 12 would not allow a BD disc with a bitrate that high. Perhaps S14 raises the ceiling, but that would not improve the image quality observed from video created with most more modest models. The Panasonic GH1, for example, shoots 1920x1080 24p at a maximum 17mbps.
Using Studio 12, one can chose a "custom" bitrate and lower the ceiling as low as 5mbps. That would be suitable only for a very, very long standard definition project rendered to BD media. In the case of an HD project, a bitrate below about 10mbps will start to feature lots of "mud" or pixelation in the action shots, but could be OK for an HD slideshow.
If your project is mostly still "slides" and the video is SD, has little action, or was shot in poor light, a bitrate setting between 10mbps and 15mbps, allowing up to nearly 4 hours of video on a BD SL. If your video lasts two hours or less, it is best to use the "Best Quality" setting, even though the actual average bitrate may be below 20mbps.
When creating BDs, I've not stress tested the MPEG and AVC format options against eachother under every possible condition. However, I note that the AVC uses slightly less space and produces a slightly lower average bitrate, for whatever custom setting one uses, compared to the MPEG one. The AVC playback output seems not to get above 19mbps, even if one selected the "Best Quality" setting, which might explain why it uses about 79% of the space that MPEG does at the BQ setting. AVC does take longer to process.
The Studio 12 disk-o-meter overstates the amount of space used. In practice, one uses only about 18GB of disc space when the disk-o-meter claims that all space is used. One can put more on a single-layer disc but designating the target media as a 50GB DL, using the "create image but do not burn," and then flipping the target media back to 25GB SL when actually burning the disc. However, don't try to put more than 22.5 GB of video onto a disc image, or it will exceed the capacity of the BD SL media.
Most commercial BDs have an average bitrate between 17mbps and 28mbps. Some will have peaks over 30mbps. However, in practice, the maximum quality one gets off a consumer or prosumer model with an ordinary sensor is probably at about 17mbps. This is true whether the format is 1280x720 60p (actually a very good format), 1440x1080 60i, or any of the 1920x1080 formats. Anything over that is, like the 12MP photos shot with small 1/2.3" CCD digicams, not perceptible, except in terms of memory use.