I'm not sure about the idea of "a warp portal"
I agree in a roundabout way with what you're saying. From my perspective, warp portals are typically portals to the warp, not a portal to another location in realspace. So, for instance, enslavers create warp portals. Daemons might use warp portals to gain access to the material plane. Chaos marines might be inclined to travel using warp portals. But "stargating" from one end of the galaxy to another sounds more like a webway (whether it's an Eldar gateway, or one of the truly ancient ones that were constructed by the Old Ones. The latter might explain why the gateway has been forgotten by the likes of the Eldar, and why it is now open for exploitation by the likes of the Tau.), or perhaps a necron teleportation gateway. (As per the second Ciaphas Cain book. He calls it a warp portal, but the idea of necrons using warp technology never really sat right with me. The necrons predate psykers, and the warp as we know it. When the necrons were young and developing, the warp was a calm ocean with no ripples or currents. It was the creation of psykers that turned it into the turbulent storm that allows FTL travel. I like to think that necron teleportation gateways, monolith portals and personal teleporters use entirely realspace based tech, but I'm getting sidetracked here.)
It could be that Velterax is a warp hub, sitting on one end of an exceptionally fast and stable warp passage. A good Navigator can make the journey in an astoundingly short time, and the passage is even usable (if not quite so quickly) by the psychically blunt Tau.
Of course, this would also result in the system being quite a major space port, but perhaps one of the other worlds in the solar system serves that purpose.
Sounds very similar to the Levitus gulf stream, which connects the Cadia system to the Levitus subsector, which lies several lightyears from the fringes of the Imperium. This is, of course, where most of my games are played, as the region's isolated geography gives us a lot more freedom in various regards. The gulf stream is a one-way trip, however. Going from Cadia to the Levitus subsector takes virtually no time at all, but the return journey can take years. For this reason, it is not considered to be athreat to Cadian security, since things can only enter the gulf stream aperture, rather than come out of it.
Just in case that offers any sort of inspiration....
