Hopefully not to the point that they become very overcrowded! I can see ScotRail needing if not an all 2+5 fleet perhaps a good number of 2+6 sets.
AIUI (from asking on here a while back), the refurbed 2+4 sets will only have about as many standard class seats as a 170 (14 more, perhaps?). So yes, they're gonna be overcrowded, unless you're in First.
I think the rationale for having the 2+4 sets was that that is all that will fit into Queen St until works are completed there... hopefully once that is done they can all have at least 5 coaches.
Re the choice of Scotrail or LNER at present, there are lots of factors. Sure, there has to be a train at about the right time, but I often have an hour's flexibility, and this often gives a choice of carriers. The LNER HSTs are *much* nicer than 170s[1], partly because of the interior but mostly because they are less crowded... and that is presumably also the reason that there is a huge price difference at present. When booking advance tickets between Edinburgh and Aberdeen it's usual for Scotrail to be £34 or £55 and LNER to be £15, and that's a *very* significant contributor to the choice of trains!
I don't see the price difference changing markedly - if 9-car trains from London run all the way through, they're always going to have plenty of empty seats for the last part of their journey, and hence lower prices are sensible - so if I'm still living in Aberdeen by the time any 800s actually appear, I'll probably find myself using them.
[1] And they're also much nicer than the GWR HSTs, because of the GWR seating layout.