Good question. The basic difference is that the 627 has a bottom discrimination mode and thus is limited to the transducers it is programmed for. When used in the fish finder mode both the 620 and 627 have equal preformance.
How well did the 600L perform at 600ft? A 600 watt ducer has about a 40 degree beam on 50Khz. The FCV-600L was also a 350 watt machine. Both the FCV-620 and 627 have more power, 600 watts. But if you put a better transducer onboard you can increase the sounders performance even further. A better transducer will have a narrower beam width which will focus the sounders power into a smaller area producing better fish returns.