My doubts with that is that the RAAL upgrade is 700$ for the towers, and that's basically ignoring/replacing the NrT crossover with the RAAL crossover, in other words, you're only purchasing 2 RAAL tweeters (so around 350 each) for the towers as the price of two crossovers is already included in the original price of the NrT towers, so the RAAL upgrade cost is majority tweeters imho.
So if the upgrade is for the Sierra-1 RAAL tweeter, your current crossover is useless, so you must buy two more crossovers, so if the pricing is similar, you're looking at >700$, not below 800$ in my estimate... Which is a significant cost! It's the original price for the Sierras!
If we take the NrT upgrade, it was 340$, so I'm not sure how great the pricing works.. Sure, maybe volume discounts and all will factor, but a RAAL upgrade will be quite costly... There are cheaper ribbon tweeters available though, some which would probably fit in the existing cabinet...
But thinking back on my earlier guess, my mid cabinet option might also be quite expensive, not sure how much it would cost to make 2 small cabinets + two mids + crossover... Maybe also costly... And performance wise, not having heard the NrT towers or RAAL towers, I could only guess which of these two upgrades would be better...
Another thing, the towers use different bass woofers from Sierra-1, so again maybe another component to change? Leaving as is would be ok too, but if you add the mid.. Anyhow, can't wait to hear more details