B-frames quality questions (x264)

[andiandi]

I was wondering if we could improve quality of b-frames without causing any issues, nor increasing filesize so much if possible/realistic. I'm aware that we don't watch a video frame by frame, but it still can be important with some videos in which pausing frequently on several moments is useful (e.g. family tapes...), or, if we want to take screenshots...

The thing is that I don't want to use custom settings without any advices, so I have some questions (note that I'm usually using 5-6 b-frames along with b-adapt 2 and b-pyramid) :

1) If I change ipratio value, would it change the quality of EVERY b-frame or just those which are using pframes as reference ?
Or do I HAVE to change both ipratio and pbratio values so that all b-frames would have a quality improvement.

2) I read here and there that pbratio doesn't work if mbtree is enabled, so if we use Grain Tune for instance, which enable both ipratio and pbratio, will mbtree be ignored if it's enabled ?
If yes, do you have any advise on how to "compensate" compression loss (e.g. in high motions) due to no-mbtree ?

3) I just want to have a close quality between b-frames and default "medium preset" pframes (or at least, not too much lower). So, can I achieve this by changing a bit ipratio and pbratio, or do I have to choose more "extreme" values just like Grain Tune do ? (Grain Tune is just an example, my goal isn't grain retention).
Note that I'm not bothered by low quality bframes in high motion moments, but rather in low motion moments.

4) If we change ipratio and pbratio, is it a good idea to decrease psy-rdo (the first value) to compensate ipratio/pbratio bitrate consumption.
Or maybe you have other advises to improve compression ?

Thank you