My initial thought before any testing on this is the following- Gastra needs as much STP as you can to get to Trueflight (and any TP overflow is just gravy for that weaponskill) as fast as possible. While you might notice a difference in your white damage, no other ranged weapon has such a disparity between TP phase/WS phase (maybe Death Penalty, but this is a Ranger Forum), and a little more on your crossbow bolt shots won't change this. You'd have to probably multiply the damage dealt by TP shots by a factor of 5 to come close to seeing something worth gearing for.
Now, I do think the JSE neck might have a role in Annihilator and Armageddon builds, both of which deal better base damage before AM, and will definitely deal a helluva lot more damage with the AM up over Gastraphetes.
In regards to "is 25 AGI worth more than 13 MAB"? (JSE+2 neck versus Baetyl Pendant) Its gonna depend a LOT on your current Trueflight Build. If your build is hurting for MAB, the Baetyl pendant should win out easily. Otherwise, you probably will see a slight boost with the AGI.
And most top-tier Rangers are actually hurting for MAB these days in their Trueflight builds with the introduction of AF+3 head and Relic+3 legs- two great WSD pieces but both have zero MAB. Our sets (best in slot that is) prior to the release of these pieces included Herculean head and legs augmented with MAB/macc and WSD, so the loss of those two pieces probably means a loss of around 40MAB to gain 12-16 WSD. A good trade, but it makes high MAB in the waist,ears,rings, and neck even more valuable.
I think an interesting debate will be once we see some fully augmented +2 Ranger Neckpieces and compare them against Fotia for Last Stand. That's some serious AGI to put into the number crunching.