So even if you fuck up and Taunt as they use Magic Coat, you lost nothing.
Is this CB Genesect you are using (which changes the dynamic since yes, some people still decide to give you a SpA boost) since iirc U-Turn from Tournadus-T + Genesect doesn't OHKO (and I think thats without Tanga Berry). If I didn't fuck up my calculations (which is possible I guess) then your above point is incorrect as Deoxys-D just got 2 layers on you (you taunted, it magic coated, set up SR on U-Turn, then spiked on U-Turn. and thats a win in my book. its still a 50/50 I think tho, when leading with Tornadus-T, but its still going to get SR up most of the time. Also, U-Turn doesn't really cripple Deoxys-D (from Tornadus-T), iirc it does around 25% or something, which enough for a player to make a judgment call, switch out Deoxys-D, and bring it in on a slower member of your team anyway.