In case you have an Epic Tune, they also offer a custom ratio DCT tune:
LINK.
I haven't tried these myself but did have a 3.45 ratio on a DCT car in the past and felt like the DCT tunes were a little off on the shift timing, glad they appear to have solved this now.
Of note, I just installed an OS Giken diff but with the standard 3.15 DCT gearing, will be trying it out in the next few weeks with the OEM BMW GTS DCT tune. Likely good since I didn't change the ratio.