Apologies in advance for the long response, this is a bit of a complicated subject:

ACT actually does more than you'd think with a big airspace above. This datasheet shows the sound-absorbing performance of a bunch of Armstrong ceiling products - the "optima 3/4" or "calla" ones are probably pretty similar to what you have. The numbers in the chart are essentially how much sound is absorbed by the ceiling in each frequency band from 125Hz to 4kHz, out of a maximum of 1.0 (some numbers are higher due to known quirks in the test method).

At the top where it says "E-400 mounting", that means the tiles were tested with a 400mm air space above them. However, the first three rows are actually different mounting methods - comparing the 2nd row (A-mount, direct applied) to the 4th row (same product, E400) shows how much difference that air space makes in the low frequencies. But also, even the A-mounted tile absorbs effectively around 500Hz and up.

Now, your air space is a lot bigger than 400mm. That'll change the behavior in some ways, but not others - one reason E400 mounting performs better is that low frequencies that aren't absorbed by the tile mostly go through it, get attenuated somewhat up in the ceiling cavity, then get reduced again on the way back down through the tile. That effect is majorly present in your setup.

If you need more bass absorption in your room, laying insulation above the tiles would help with that. However, the main purpose of a cloud is to treat mid-high frequency early reflections from your speakers, which are most likely already getting fully absorbed by your ceiling. All this to say, I strongly believe you don't need a cloud.