I didn't realize they don't have a 240w single channel. As far as whether you need the channels filled I'm not sure, but if the wattage is split per channel that extra 40 watts won't matter since only 40 watts is being allocated per channel.
Consider using HLG-120, 2 per lamp. At 65% capacity they would still be over 92% efficient. The drivers can be wired to a single dimmer.
If you used the Inventronics 320w at 60% capacity it would be 90-91% efficient. Not bad and your only single driver choice at the moment.