My 535xi seems to have good power taking off through all the gears yet if I'm on the freeway and punch it (WOT), it hesitates, stumbles and goes into limp mode. I get a yellow outline of an engine in the upper right hand corner of the dash screen for a while then it goes away until I try it again. The dealer is telling me that the plugs are completely worn out (58,000 mi) and that both turbos need to be replaced at about $5300.00.

Does this sound right?

 

This thread might have some answers. Although it is not the xi, which you have but may have some information that could help you.