I saw some posts on another board (will look for them) where the poster had checked part numbers and the proportioning valve was the same between drum/disc models. In fact I remember the post going on to say that the mechaninsm in the drum does the proportioning / fluid quantity job, thereby allowing the manufacturer to keep more parts consistent.
I too have spoken with a rally team mechanic who had a customer do the same swap in a '02 OBS. This customer did ice racing and commented on the better brake modulation with the discs.
But all the info in this thread is second hand or speculation. That being said, I plan on doing the swap in my TS.