I agree that you'd very likely hear a difference. The only problem is that I'm not entirely certain that the Sierra-1 NrT speakers themselves are the current cause of the sounds you find a bit objectionable. I have to say, I'm rather surprised to see the Sierra-1 NrT being described as "a bit too bright", and lacking some mid-range clarity. Just based upon the inherent sound quality characteristics of the speakers themselves, that doesn't really jibe with the sound they're throwing out at you if you were in, say, an anechoic chamber, a sound treated studio, or a wide open field
So, that leads me to believe that the source of your issues are actually part of your current room acoustics.
Here's the thing: the Sierra Towers with RAAL do offer better mid-range clarity, and even better transient response and clarity of detail in the treble thanks to that fabulous RAAL ribbon tweeter! Also, and I think this might be more important than even those factors, it has wonderfully even and controlled polar response, with the off axis response of the RAAL ribbon perfectly matching the off axis response of the mid-range driver. The reason that is so important is because we use the reflected sound off of the side walls of any room to subconsciously get a sense of the room's size. And if there are anomalies and unevenness in the polar response of the speaker, that creates confusion in our brain that causes fatigue, and ruins the illusion of realism. It's a big part of why we can easily tell we are listening to speakers rather than a live performance. The Sierra Towers with RAAL excel in this area, which means a greater illusion of realism, and less fatigue.
So, plenty of good reasons still to upgrade. BUT, because your description of what's bugging you about your Sierra-1 NrT speakers sounds so much like room acoustic problems, I really think you should take a moment to address those issues first! For one thing, fixing the acoustics might make you perfectly happy with your current speakers, and remove the desire to upgrade! And second, it isn't really fair, in my opinion, to buy speakers as wonderful as the Sierra Towers with RAAL, only to put them in a bad acoustic environment. It's the proverbial, "buying a Ferrari to drive in stop-and-go city traffic".
So, a couple of things:
1) tell us all about your room: dimensions, layout, materials, where the doors, windows, and other openings are, and where the furniture is placed. We might be able to spot an obvious problem and offer solutions!
2) talk to Dave and/or the other fine folks here at Ascend. Get their input, for sure.
3) I highly recommend making use of the FREE room analysis service from GiK Acoustics. You can find it here:
http://gikacoustics.com/acoustic-advice/
It's free, the folks at GiK are total professionals and awesome to deal with, and you'll learn a ton about your room, even just from filling out the form and being forced to think about your room acoustically!