Regarding "classifier" most of the time is going to be only one (evaluate the classifier performance of one model), but I want to leave it open to handle multiple classifier results.
The "c_modeled" set of values is also be very small since it is the classification output for the model (e.g. 2 for positive- negative classification).
As my reply on the first comment, yes I would use MERGE on the TABLE operation over the non-sorted "compared" recordset.

coi_tot:= TABLE(compared, {classifier, c_modeled, totPos:= COUNT(GROUP, c_actual = c_modeled), totNeg:= COUNT(GROUP, c_actual<>c_modeled)}, classifier, c_modeled, MERGE);

Does FEW do a better job than MERGE for this case in the TABLE operation?