I don't have the same aftermarket dash that you guys do, but I do have a 4th gen with the factory speed sensor working.
I think your problem might be related to the number of signals sent by each unit.
The magnet will send one signal per wheel revolution, so the speedo just mutiplies whatever you said your tire circumfrance is by the number of revs it's getting and does some math.
I don't remember the exact number, but the stock speed sensor sends something like 7 signals per wheel revolution. So, I had to tell the dash I've got (acewell) that I've got really tiny tires. It still has a cap to how fast it will count though, and the speedometer never exceeds 82mph or so now. If I moved back to the magnet, it'd read fine, but for me, basically I know I'm speeding if it says 82. . .
So, might have to do with how fast the dash can count.
Love both your bikes by the way, they look amazing.
*EDIT*
I almost forgot, my stock speed sending unit was bad when I got the bike, so when I tried to hook it up at first, I got no signals at all. It should send a pulse similar to the magnet, so, if you're not getting anything at all, maybe see if someone you know will let you borrow their speed sending unit for a bit. I think there's just two bolts and one plug that connect it to the bike.