At 5Hz you'll have a 20dB advantage over the current system with 1 1/4" less P to P excursion.
Yes, room gain is significant in your HT, but you really need to get REW up and running with a good sound card and microphone. If you don't know the transfer function of the room, you will never reach max potential.
You're correct in saying that room gain lessens harmonics as a percentage, but you're guessing without knowing the room gain at the fundamental and harmonics frequencies of a given frequency or bandwidth.
Your silos are so big that you would not need stuffing with 2 drivers in each. As you know, I'm no fan of huge containers. They limit upper BW headroom for not much in return. But, you have the power plant defined at 600W each driver and that's well within the limitation.
Remember, the SPL meter is recording peaks of the sum of all simultaneous content, or total sound pressure level, so it's impossible to tell if it's registering the fundamental, its harmonics, the rest of the 4-5 octave spread of frequencies, a huge peak in the in-room response higher up or all of the above. This renders correction factors useless.
The SPL meter measures Sound Pressure Level at any given instant, nothing else, and quite inaccurately at that when you're talking about single digits.
A FR graph, close mic'd, compared to a FR at the LP will give you the math you need to fashion EQ and predict the output/THD capability of the system.
In any case, it would be a marked improvement, no question about it.
The cost is irrelevant, but many insist it is an issue, in which case, I fail to see the doubling in price. The original 1X18" silos cost $325 each. Adding a driver to each would only entail the cost of the drivers because the $$ spent building the containers remains unchanged.
I'm in the dark about amplification. I know you've said you have 600W to each driver into 2 ohms, so I assume you have the dual 4 ohm drivers a 4 channels of amp. I thought I read you were using ADA MP-502s, which are pretty spendy IIRC. I just don't know. But either way, you could use a single $800 amp to power all 8 drivers.
The total cost would be a 50% increase, not double.
6dB headroom above 30Hz, improved transient response, 20dB advantage at 5Hz with much less excursion (distortion), tunable F3 and F6 BW and longer LF system life.
All in all, the most cost effective upgrade you'll probably ever make to the entire system, IMO.