You are assuming that low passing the mid is an upgrade, in the case of the fEARful plans using the 18sound mid, it maybe for some. For others it may not be. A different mid driver will have different requirements, and will meet different tonal goals.
You are still assuming they two different lines need the same thing. They dont. They are different goals. What you are asking is along the lines of, "Will these halogen bulbs that are an "upgrade" on my honda be an upgrade on my dodge?"
Having sat down with a 12/6/1, a F112, and a Bassic 12, I can tell you they are all different, and I walked out of there with a pair of 12/6/1's.
I wouldnt call the t3w an improvement. It is a different design goal. A lot of the tonality is covered by the mid driver. The low end character is controlled by the porting and cab size. Since both of those are different, you get a different sound. The cross overs are the icing, and while they can change some stuff, they can only shape so much.
Honestly I dont think cabs with mid drivers are for you. If you didnt care for a fEARful, regardless of which cross over is in it, then I dont think you will like the fEARless line, and probably not greenboys line of cabs using full range speakers. There are a lot of guys on this forum that dont want to hear mids, thats ok. There are a lot of guys here that dont want lows too, and others that just want to play low mid guitar tones with nothing over 2k, and nothing below 100hz.
Just different goals.
Look at it this way, you didnt previously want the low end content in your bass rig right? but now you want to double on kick? That is going to require you to shape the signal from both sources differently if you want to keep the tonality you have now. Doing so would probably require some level of DSP or something else. With that in line it wouldnt matter what cross over you use, because you could adjust it.
