It's not " very uncompetitively priced" at all. I paid £17 for 5 litres of the stuff and bare in mind the BMW stuff is down to -63 which means you can dilute it many times more than the 3rd party guff you buy, meaning it goes much further.
And yes 3rd party stuff will lead to sludging in the tank and pump, which are both difficult to get to.
I'm sorry to burst your BMW bubble but in fact as far as screenwash goes it is fairly overpriced.
You can pick up 20L of VW/Audi screenwash - go check out detailingworld on how good it is and that costs £13 for 20L! It is rated to -70C!