The Elmar ƒ/2,8 could use the barn door hood that fits Summitar, Summicron and pretty much any 50mm lens that got 42mm front, that includes Zeiss Sonnars and the FSU Jupiters.
As far as performance @ ƒ/2,8, IMHO the Elmar is more efficient lens design, since it sports less elements.. although its not as symmetrical as Elmar ƒ/3,5.
ƒ-stops describes DOF better than light transmission. Thats why cinematographers use T-stops.
ƒ-stops does not take in account the number of lens elements, surfaces etc. etc.,
The only thing Elmar and Summicron share at ƒ/2,8 is the same DOF @ ƒ/2,8 (or any other ƒ-stop), character and light transmission are gonna be different, hence some folks swear by Elmar, others by Summicron.