but it never sounded that great due to the heavy cone I think
It would be really nice if you new the T/S-parameters of the PD.
Now it's difficult to say whether you didn't like the driver, the cabinet or the incorporation into the rest of your system slash processing. If you know the T/S-parameters it's easy to compare them to the 21PW1400Fe and say if it's the same or completely different and if it attributed to your experience of the Cubo 21. To answer your question whether this is going to improve things for you without the T/S-parameters, we need to figure out what you didn't like about it exactly. What did (or didn't) you do to it, processing, amplifier power, what tops did you use (any delay on those), magnet inside the chamber, magnet inside the horn, etc. To me the 18LW1400 in Cubo Sub with the magnet inside the horn or inside the chamber sounded like two different cabinets, one for 'rumble' and the other one rather tight and loud.
It would also help if you have heard the PD's in another enclosure, anything that tells us if it's the driver, Cubo or system settings, or a combination of those.
Typically a 21" driver doesn't perform that much better than an 18" driver, in the same sized enclosure, it just has more potential. You need either a lot of power or quite a bit of EQ to get that potential (air displacement) to fruition. Someone in the Netherlands is building a custom version of it and probably going for the RCF LF21N551 or Lavoce SAN215.30, I assume with a small nuclear reactor to power the amplifiers because they can handle it and need that kind of power to really shine (beyond the scope of an 18" driver).
Best regards Cubo