Hi MBH,
I'd agree with Tezzer, I've done through tenons of 5mm in 12mm timber without a problem, I would think provided you are set up correctly (dead straight cuts), 9/10mm would be fine as well.
Tezz, don't know what you think, but could it be worth making the tenons yourself to say 40mm, depending on the position of the joint?
Regards,
Albert