turbo cars gets very complex when you have to build a high performance car. It's not as easy as just dropping a larger air-air intercooler b/c there's pressure drop with that system. You'll likely need to run shorter pipes and higher boost, which then creates even more heat...and the cycle continues.
BWM could have just a failure of the eqiupment instead of the actualy setup? I would think designing a turbo system is pretty easy, I mean, turbo engines has been around for about 90 years in one form or another.
FWIW, the early Evo 8's also at one point had a water sprayer on the intercooler (debatable performance gain however).