My buddy has been waiting for his Jacoby black for about 3 months, and it showed up yesterday. He brought it over to try it out, and I am VERY impressed. I think it's about a 12.3mm?, threaded for a 3/8x10 pin to fit his McDermott cue.
We did some deflection testing alongside my Lucasi 11.75mm LD maple shafts and found the deflection to be identical. Not similar, but identical. I found it pretty crazy that the Jacoby with it's larger diameter shaft has such small deflection compared to mine. The taper was similar on the 2 shafts, a little longer on the Jacoby (shaft is thinner closer to the joint than on the Lucasi, think they are both "Pro" taper).
I really wanted to punch holes in this thing, but I can't. Seems like a great shaft. Super smooth/slick, to the point of almost being hard to hold on to. Super stiff, but you don't seem to notice it until the follow through. It's not "fragile" feeling like most really stiff cues. The fit and finish was 100%, I could find no blemish anywhere.
Not sure what my point is, just wanted to throw up an initial first impression from an impartial 3rd party. Hopefully it helps someone that's on the fence, one way or another. Bit too rich for my blood, but I'll be keeping my eyes out for a used one in the next few years.