Turbo failure would be the least of my worries of actually happening. If you bypass the BMW dealership way of fixing things then what in the DEF system is actually expensive to replace? Even better question if an expensive item in the DEF goes out then will you actually try to have it fixed or bypassed? You have to do what ever makes you feel comfortable though. I plan to get the cheaper of the BMW warranty offerings for a piece of mind over a catostrophic failure. I might change my stance since I have 22k miles or so to actually decide.