I have the big ally cases from KTM on my 1190 (Re-badged Givi Trekker Outback) and they have loads of room (as do most the metal offerings) but sit a long way out from the bike. Handy for us when we go touring as the Mrs will only have a top box on her bike so I have to carry most the stuff.
I have not noticed a stability problem, it once got a bit weavy at around 110 on the Autobahn, changing from Comfort to Streey damping solved it - or at least up to about 120 as I did not go any faster than that, not sure at what speed the luggage would get ripped off by the wind resistance and not keen to find out. The 1290 is far firmer to start with so would expect it to be as good or better.
The KTM plastic stuff looks OK to me, and if I only did solo trips would be more than I need, but the exhaust side box is pretty small if you are working on a 1 pannier each system with the Mrs. You could still get by - I managed on the GS with two-up trips and the Vario luggage (about the same size)
TBH even with the KTM Plastic luggage the bike ends up very wide so filtering is limited by both options, if you like the Bumot stuff then I would say go for it, if you like doing 150mph all day long I would suggest you sell me your new bike for £10k and go and buy a ZZR / GTR 1400.