To be clear, is your main reason for doing a 9.5G batch to reduce headspace in your carboys? I only ask because I've done a few batches in which I split 5 gallons into a pair of 5G carboys -- 2.5G in each -- and tested different yeasts in much the same way you are looking to. My understanding is that the CO2 produced during fermentation will fill the additional headspace, negating the oxygen issue.
I'm sure someone more experienced will chime in and correct me if I'm way off base!
Fermenting: American Stout
Kegged: Biere de Garde, Premium American Lager, "May Day" Belgian Pale Ale, "Witty Retort" Light RyePA, "Spring Break" Marzen
Bottled: "Sh!tshow" Schwarzbier, Maibock, Saison, Kolsch