Disc Cover on Road Bike

Same benefit that it would provide on a tri bike.  Nothing silly about it at all, and it's generally believed that the aero advantage outweights the weight disadvantage in nearly all cases. (And Florida is definitely not one of those cases.)

The aerodynamic benefits exist at any reasonable speed.  At lower speeds, there is less drag to be reduced (thus less overall drag reduction); however, you get that 'smaller' benefit for a longer period of time. (over a fixed-length course).  At higher speeds, you see a greater drag reduction, though not for as long. (Since you're not on the course as long.)  A win either way.

But if you're at front of MOP, you're likely going plenty fast enough to 'see' a benefit.  Of course, just to be real for a minute (blasphemy!), you're probably talking only 5-10 minutes over the course of the IM bike.

Thanks all, Yes I bought aerobars when I bought the bike so that is covered (they actually have not been off the bike.)

Based on the other responses, I think I need to make a run up to Excel Sports in Boulder.