When FDCL's proc was just a regular mind spike that didn't wipe dots - it was trash, but when they made FDCL give MsK's + 50% bonus damage, I think that ended up taking the DPET well above Mind Flay.
Just punched a dummy for a few casts, with 23,421 spellpower my average FDCL non-crit was 60k, my average mind flay tick was only 17k, mind flay ticks three times in 2.4 seconds, FDCL proc is a single GCD, so MF's dpet is like 25,500 versus FDCL's 60k (assuming we're not gcd capped yet for that dpet). A 25% buff to MF would bring its dpet to 31,875 - but still way below the like 60k dpet on an FDCL proc - it's still going to always be the right call to ùse FDCL procs when you have them over Mind Flay, even with a 25% buff - hell we could double MF's damage and only then start worrying about FDCL procs not being worthwhile - MF apparently blows right now!