Actually I've gone the other way! Out of the 7(?) audaxes I've done so far the first 6 where done a variety of recumbents, however the last one was done on a Fixie and the 4 audaxes I'm doing this year will all be done on an upright! However this switch was mainly due to getting very annoyed with both the EU distributed of my recumbent, and then the recumbent itself later on. I suspect I'll be switching back to the recumbent for the 1000km and 1400km events I have vaguely pencilled in for 2012 and 2013.
As for speed. Well it's often said it takes about 6 months to adapt to riding a recumbent, and depending on your recumbent the extra weight of bike you may be pushing (some 'bents are better than others in this respect, eg: Bacchetta Carbon Aero or Challenge Fujin SL2 but they have ways of making you pay for it!) will make you slower on the uphills. I've also seen comments from people who where faster on an upright simply because they could draft in a group, as opposed to be by themselves all the time.
On the plus side, when/if you do longer events than a 100km then the comfort factor might come in meaning you can spend longer on the bike. Someone who always used to finish lots of Audax events in the UK used to claim that uprights where faster below 600k and recumbents where faster above 600k for that reason.
YMMV of course, if things go to plan I can give you some better figures in 2013! :-)