Having owned an Syb-5 and pretty much every bass synth pedal (and selling them all), i think the tracking on the Syb-5 for most notes, even though it does "track" down to A0, is pretty glitchy with some latency.
It's a totally valid wish for the Axe..i've been asking for it since the Axe II and would love to see it. If the C4 can do it then i'm sure the Axe can too.
The other thing to consider is how the actual synth tones are being created. From what i can tell, the Axe and C4 both sound like they're producing actual waves in their synths which then need to be triggered by a pitch tracker of some kind. Some of the none hex pickup Boss synth stuff (SY-1 for example) seems to be more of an "input shaping" kind of synth sound that often has unwanted overtones or sounds more organ like.
It would be great to see how all synth pedal manufacturers are achieving their pitch tracking, but i'm sure these are all kept behind the scenes for obvious reasons.
The Pigtronix Mothership models were both analog synths with a pitch tracking circuit..and these did track the low B0 of a 5 string bass but with some glitches. The C4 is also glitchy sometimes. The Eventide H9 could also track 5 string bass in bass mode but with an unplayable amount of latency.
So i think as
@Admin M@ says above, the laws of physics mean that the lower in pitch you go, the slower the tracking is going to be, so i think each manufacturer will decide at which point/pitch the tracking is no longer acceptable and will cut the tracking range there. The workaround i mention above with the Pitch block does have latency but is still usable if blended with a direct or distorted sound.
For bass guitar synth tracking, i still think that a polyphonic pickup (Roland, Cycfi) is the best way to go for several reasons. You don't get the glitches from one string to another on monophonic lines and, in the case of the Roland/Boss hex gear, each string has a certain tracking range. So currently, the best possible setup which would be very expensive, would be to run a multichannel pickup into several C4 pedals (one for each string), to get the most up to date bass guitar synth.
I've mentioned this before on the forum, but a future modular Fractal product that would allow the user to have any combination of fx blocks (eg 5 Pitch blocks and 4 Synth blocks, but less Reverbs for example) and a multipin input, would be totally killer. Then you could assign each string to it's own Synth block and specify the tracking range of each Synth block.
For now though, the C4 is your best bet.