Wouldn't the speed gained from the start finish line to the speed trap at the end of the straight in Q be an ok-ish measure for that? Of course there's other factors which contribute to the speed change but it's an indicator at least.
data from here, speeds in kph
https://www.fia.com/sites/default/files ... ds_v01.pdf
delta top line
MAZ 30,3 341,1 310,8
SCH 29,5 337,8 308,3
SAI 28,7 340,9 312,2
RUS 27,9 341,9 314,0
LEC 27,6 335,8 308,2
KUB 27,1 340,8 313,7
GIO 26,9 342,3 315,4
STR 26,5 338,4 311,9
LAT 26,0 344,6 318,6
TSU 24,5 332,1 307,6
VET 23,9 334,4 310,5
BOT 23,8 338,3 314,5
NOR 23,2 341,4 318,2
OCO 23,0 335,8 312,8
GAS 23,0 334,3 311,3
HAM 22,9 338,5 315,6
RIC 22,0 340,2 318,2
ALO 21,3 339,9 318,6
VER 18,8 334,6 315,8
PER 18,4 336,4 318,0
They gained (by far) the least.
Other than that it would be hard get reliable data where they were running with opened DRS without a slipstream, closed DRS with no slipstream is easier - that is if one could get the data in the first place.