Those are all great points, Lelle. I didn't really think of harm being done, but I can see how that could happen. And probably the more technical stuff is not fun for most of the audience. I just wish there was more info out there on how the pros go about polishing that last 5% and overcome plateaus in tempo. Something similar to that is this clip of Lazar Berman going over rhythmic exercises in Debussy's 7th Etude.
Sorry for ressurrecting an older thread but I just wanted to add some input on this topic. I think the unfortunate truth is that pros don't overcome plateaus in tempo, because they don't need to. They've mastered the craft to the point that playing as fast as is needed isn't difficult. Yes you might have to practice slowly or in fragments for a while to learn the notes and the patterns but that's it. Plateaus in tempo are caused by suboptimal technique and that's pretty much all there is to it. There is no quick fix to suboptimal technique other than working on your technique, preferrably under the guidance of someone who knows what they're talking about until it's not suboptimal anymore. And if by polishing you mean polishing cleanness of playing, with better technique the less polishing is needed and the rest is taken care of by practicing so carefully and/or slowly enough that you don't make mistakes, and then you work 6-8 hours a day like a number of concert pianists do.Someone can challenge or correct me if I'm wrong but I think there is not much more to it.
My guess though is that "overcoming" those last technical hurdles has less to do with dexterity, and more to do with having an extremely clear musical vision for each note. And going back to the original post, this is probably the reason most masterclasses do just focus on interpretive issues instead of mechanical ones.
The reason teachers focus on interpretive issues in masterclasses, based on what I know, is because there usually isn't enough time to rework someone's technique in a masterclasses. That might take months or years, and so it's impractical to dwell on that. If there is a simple fix, teachers will mention that. However, most technical issues go deeper, and most people (although there are exceptions) aren't quick enough to understand a technical concept in 20 minutes. So teachers work on interpretation instead. However, the actual process of improving technique isn't about having clear musical vision in many cases, imo.