As far as unknowns-you can talk to tuners about any questions you might have. I talked to my tuner to make sure every fault and diagnostic function would still work, and was assured that they would. I had a few other questions for him at the time, and he was happy to answer them.

Personally, a total outright cost of $2000, making my vehicle hardy for the long run, is worth it.

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'10 X5 35d- 279k mi, deleted and DUDMD stage 2, daily and tow vehicle
'01 330i- Drift car: SLR Super angle, BC coils- keep it simple
'07 335i 6mt - Sold @ 171k miles- stage 2 daily commuter for 2 years