Generally most all of the experimental AP's on the market today will smoke the STEC in performance. These modern all digital AP's and their servos are simply amazing to experience compared to the old school rate based units.
As for your other questions: It depends
Some AP's like the TT Sorcerer can couple to both GPSS/GPSV, NMEA and NAV Radio signals (VOR/GS/LOC) others that TT make can only talk NMEA or GPSS/GPSV.
Some EFIS units connected to the proper TT AP can transfigure the NAV radio signals and convert them to GPSS/GPSV that the TT AP can understand and be able to couple to VOR/GS/LOC signals.
Then there is the AP's that can be built into the modern EFIS systems. Feature sets vary between EFIS makes and they are constantly updating their capabilities so it is hard to keep up with which one can do what.
The Garmin G3X AP can couple to it's VNAV just fine. It can also pretty much couple with any other navigation source that it supports. I imagine this will not change when they add the ability to set your own glide angle.
Last edited by Brantel : 03-30-2015 at 12:16 PM.