I don't doubt that R27's and R30's ran on the C, but not in 1987 or 1988, as you claim. None ran there until after Archer Ave. opened on 12-11-88. Perhaps your dates are incorrect. From personal and extensive observation from the time periods in question, I can say they are.
Believe me, if I saw an R27 or R30 on the C line at the time you claimed they ran (1987/88) there (and I would have, since I rode the lines more than you, a mere commuter to/from work did), I would have taken notice immediatly. In fact, any time these cars did NOT run on the eastern division BMT lines during these years would've been an absolute shock to me.