My understanding is that the AR side Turbo button is for DIN connections to other devices with which it can negotiate a speed, be it the TM1 or A4 etc
To get the faster transfer speeds you use the Turbo button and tick Top two sds related checkboxes too in C6, make sure Midi I/o is USB only
Works fine for me, no need to check the uncheckable Turbo box in the AR
Only just tried it, so may be missing some nuances fwiw