So the TRT's are relatively more cost-effective on the bigger speaker.
Don't sell yourself short. If a $1200 upgrade improves a $3800 speaker and the consumer is happy, it is money well spent.
Here is an example. My most popular modification lately has been EXtreme modifications to the ASL Hurricanes.
Cost of brand new amps = $5000
Cost of EXtreme modifications $5100+.
Every customer to date has been "Extremely" happy (sorry) with the modifications and all felt it was well worth the investment. So, if the $1200 cap modification brings a justifiable improvement to the RM30..........
For about a 25% increase in price I am wondering if I will get a 25% improvement in performance
There is no way to define "25%" improvement. "Improvement" is like "opinion", very subjective.