I didn't figure the question was about tuning the DCT, rather what ESS had available for the E89 w DCT. I asked them the same question directly and got the below response. But your r right, it's odd that they call it the stage 1 when it makes the same power as their stage 2 for non DCT trannies.
Per Asbjorn w ESS-
"The DCT version of the Z4 3.5is can only take the power of the Stage 1 (400chp, ~350rwhp). Any more and the DCT clutches can not handle it in the long run."
'15 3500 HD Dually