A pair of 485s sound better, are less complex and less expensive to power and are less expensive in the context of the total tower package. The reason to go with the 3-Some is primarily for the reduced width. The 3-some/Syn6 combination gives up nothing in amplitude and perhaps is a bit louder.
While the HU power seems to be adequate power now, once you add a subwoofer you will find that external amplifier power for the in-boat coaxials is absolutely a necessity. You can run both the subwoofer and four in-boat coaxials off an endless selection of 3, 4, 5 and 6 channel single chassis amplifiers. Just make this a cordinated purchase between the subwoofer (voice coil configuration and net impedance plus sensitivity and power handling) and the matching amplifier. Remember that external amplifier and HU power ratings have nothing in common so the same rating in an external amplifier, in reality, is likely to be several times the power.