It isn't really a one to one comparison either. In BMWs, the convertibles have extra bracing that some people believe give the car a chassis stiffness advantage even without the roof. The one log booked SpecE30 vert that has changed ownership since it's debut at the NASA National Championships at Mid Ohio where I believe it finished 3rd and hence the new SpecE30 rule banning verts. That car was also rumored to at one point have a 170 whp motor in it which would certainly help any straight away deficit, but the fact is that the BMW E30 is a "flying brick" as they like to call it so the aero is shit on them regardless.
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It probably has a significant CG advantage as well. The roof is steel with a reinforcing structure and the rear and rear side windows are glass. All of that weight is pretty high up.

However, they both present the same frontal area and a roof usually has a less turbulent flow, at least on the roof. What is interesting is that the convertible doesn't have significant rear lift. I was surprised to see that convertible have no apparent disadvantage down the straights.

BTW, the convertible set fastest lap which is also the Laguna Seca Spec E30 track record. I think it was less than 1/10th of a second faster than Espinosa who presumably was new to the west coast track. He seems to be refining his lines throughout the race.

 

I lost my last TT session by half a second