Maybe I misread your comment that the DR1 was a worse offender. Why do you think neither one would go 115 mph at sea level?
I have reasons, but none of them are the kind of hard evidence we're used to seeing (such is WW1 aircraft performance).
I think the F.1/3 Camel did 115mph TAS, maybe even a bit faster. I recall another source quoting 118mph as the sea level airspeed for the F.1/3, but I'm not sure I could find it again. When I compare the performance gap between the F.1/3 and B2312, and if the gap were constant down to sea level, then I might expect the B2312 to be somewhere around 111-112mph.
As for the Dr.1, the best speed we've seen is 116mph at sea level with a 110hp Le Rhone, which J.M. Bruce didn't find credible. And even if it were true, the 110hp Oberusal was an inferior copy of the Le Rhone. In fact, the Le Rhone was such a great little engine that Trenchard ordered his Camel squadrons to replace their 130hp Clerget 9b's with 110hp Le Rhones in the winter of 1917. Many sources copy the 115mph figure for the Dr1, from someone who copied it from someone else, and so on...and from what I can see, the only 115mph data for the Dr1 is with that experimental 145hp Oberusal.
Anyway, those are my reasons.
------------
Nice graph wmaker!