In apheresis your blood is run through a machine and elements are filtered out and the remaining elements are returned.
I believe you can donate red blood cells, platelets and plasma through apheresis. Plasma is of little benefit in our situation (potentially even increases hct temporarily).
Double red cell donation should lower hct twice as much (nominally) as whole blood donation, but the donation interval is twice as long.
Platelet apheresis theoretically removes only platelets and as such has little impact on hct, because platelets make up only a small fraction of blood solids. However, the apheresis process appears to lower hct substantially through RBCs being destroyed by the pumping and residual blood remaining in the machine. The blood centers don't take this into account, however, and set the donation interval to 7 days!
So, to lower hct as quickly as possible through donation:
1) give whole blood
2) platelet apheresis one week later (repeat as necessary)
I started at 17.7, went to 17.0 after whole blood donation, 15.7 after platelet apheresis, now estimate 14.4, all within a few weeks. I've got a blood test coming up that I'm hoping will confirm this.
Credit to halfwit for discovering this.