I've never tried the 50GB Blu-ray, so am not familiar with any tricks/caveats to authoring to them.
What I can share is that the "Max Render Quality" button is helpful only when SCALING the output, such as 1080p to 720p, or HD to SD. If you are not scaling, don't check the box, it only adds to the render times. Also, the "Max Bit Depth" is also optional. If not working with 10-bit video (most is 8-bit) and/or not specifically using 32-bit color correction effects (and having 32-bit set up in Premiere settings), there may be no benefit for most of us to use that button. Again, may greatly increase render time with no appreciable benefit.
I have a rule of thumb when working with encoders, and that is, the DEFAULT settings are usually set up that way by the software vendor for a reason. Checking option boxes does not automatically improve things, check them only if the situation requires it and will benefit you.
About bit rate - there is a point of diminishing returns. Meaning yes, 20 will look better than 10 or 15, but you may not see any difference in quality between 25 and 30, or 30 and 35 for instance. What I'm getting at is yes the upper limit for Blu-ray is 40, but that is overkill. And the danger is that with BURNED media (versus Hollywood factory discs) is that some Blu-ray players might choke on high-bitrates on home-made Blu-rays so you are risking playback issues when "maxing out" the data rate. The same has always been true for DVD as well - many see 7 or 8 as the max, though the spec is like 9.4.
I've not made a lot of Blu-rays, but with 25GB discs, I've been able to get about 3 hours on them and I forget if the rate was about 20? Maybe 18? But I thought they looked fine, using H.264 encoding (not MPEG-2). I would of course go higher for shorter projects, but would not "push it".
Thanks
Jeff Pulera
Safe Harbor Computers