not sure what bike you have- but HED has a program that will tell you this information- for several different framesets- if you give them a call on the phone. I asked about the bike i own- a P3C- and if I recall correctly- i asked= for a 1 hour time trial, at approx 26-27 mph, with an 8 degree yaw- what's the difference-

disc- fastest
H3- about 8 seconds slower
Jet 90- about 14 seconds slower

so- basically very little difference over 60 minutes with the above scenario on a bike that shields the rear wheel very well. which lends me to believe that the time difference between bump, flat, standard discs- is marketing hooey. going to a wind tunnel and refining your position could save you minutes.