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."
'08 Alpine white E92 M3 with DTC--Sold
'12 Alpine white E92 335 Xdrive with slush box
'07 Summit white Chevy 2500 HD Duramax/Allison
'12 Alpine white Z4 35is via EuroDelivery June 4th