"I overheard a conversation the other day that included a discussion of 604 performance. If I heard correctly, to achieve the required SAR drop speeds the 604 would need to be flying with full flap / slats."
No not even close Layman. Whoever is giving you your info should be shoved in a blender. Firstly, the 604 has no slats. As for the speed - nope, doing a RAT drop test flight has the aircraft back at under 140kt with just one stage of flaps. So drop speeds with one stage of flaps, Flap 20, is no problem. BTW, there are 3 stages of flaps.