If you're looking for more par and light spread, I'm not sure why you'd intentionally decrease the power of the LX6 with HE mode. The combination of both LED and T5 will provide you with that extra par and spread.
Let it run at HO mode, add the T5 and run the LX6 with acclimation mode for at least 8 weeks. That will allow your coral and clam to steadily adapt to the increased light output. If during the 8 week period, you begin to see some light stress in the coral, then that is your sign that the current LX6 output is too much and needs to be dialed back a bit.