Does assigning a part in Sub 1, ( or Sub 2 for that matter ) remove that part from the main mix?
Yes.
Note: With the Tyros 2/3/4, the Sub out routing is only effective if a (mono) jack plug is connected to the relevant sub-outs. If NO plug is connected, all parts are output at the Main outs, even if they are actually routed to Sub outs. With Tyros5 and Genos, on the other hand, Sub out routing is ALWAYS effective, even if NOTHING is connected to the relevant Sub outs.
Best regards,
Chris