UPDATE - The Simplex+ carbon lower shaft came in and fits perfectly! I have taken it out a few times around here already and can confirm the lower-shaft-flex is completely gone (or near enough that I can't tell). A few notes to call out there though:
1) The Simplex+ lower shaft is just slightly longer than the standard Deus II lower shaft. If you plan on using the XP Deus backpack to carry your detector around, you might need to saw off a half inch to an inch from the end of the shaft. I was able to get it to JUST fit in mine by removing the little rubber stopper at the end of the shaft and getting rid of the rain cover at the bottom of the bag, but I'll probably still cut it down slightly.
2) The Simplex+ lower shaft fits a bit more snugly than the original Deus II lower shaft. It works like a charm, but I did notice it got stuck on a bit of grit after my last trip out. This could become a nuisance when searching the beach, but I suspect rinsing inside the shaft should do the trick just fine.