I have thought it would be a great work-out to have a system that would use the arms to work the front Mirage drive, while pedaling the rear mirage drive.
If you want an overall workout, pedal and paddle at the same time. You can get another 1-2 mph by paddling. Concerning speed, kayak speed is limited by design. http://www.recumbents.com/WISIL/hpb/hpbcalculator.asp
If Hobie would design an 18'-20' foot long, narrower Mirage (not with sailing capability), together with paddling and pedaling, it could give kayakers in Seda's and like racing kayaks a run for their money.