i think what they mean is, there are better drivers out there but they cost more.
The Daytons and SI's are great quality/performing drivers for their price, but they dont perform miracles. the LMS costs $850 a piece at its new price. it used to be like $950. of course its going to be better than the Dayton's and SI's. when you get into Multiples of the cheaper drivers is when you see the value but thats more boxes and bigger boxes. For the price of one LMS you can build three or four Daytons or SI's, but not everyone wants more boxes .
They all have their trade offs. all depends on your budget and needs.
as far as these being a significant improvement from your current setup. Ricci has tested the FV15HP and these drivers using about the same size boxes and same power your going to use. check out this page and scroll down to the measurements. you can do a comparison with your Rythmiks. add 3DB to the Rythmik scores and add 9 DB to the Daytons to account for multiples.
As you will see, even with no low end boost, your Rythmiks come close to the output at 20Hz, but above and below that, the Daytons take a bigger and bigger lead. After 12.5Hz, the Daytons keep going and the Rythmik's (being ported) drop like a rock. with a little boost, there will be no contest, not to mention the better coverage with more subs.