@jasonreg - I would start speed testing with the XB6 alone, with a single hard-wired client. Once you confirm gigabit there, reconnect and speedtest through the RV340, first with IPS/AV disabled, then enabled. If IPS is the culprit and you don't care so much about the speed cap, I suppose you could leave it on; otherwise, you'll either need to turn it off for full speed with the RV340 or run IPS/DPI on x86 hardware for filtering at gigabit speed. If you're still only getting 300Mb/s with IPS off, you might also try lowering the MTU on the WAN interface of the RV340. If no change still, I would factory-reset the RV340 and SG250X-48, load the latest firmware on each if you haven't already, then manually re-configure, being careful not to enable any service(s) that may bottleneck your throughput. If you fear a config change may cut speeds, confirm a gigabit wired speed test before the commit, then test immediately after the change goes live, before moving on to any further changes.
Regarding QoS, your traffic mix shouldn't need much in the way of prioritizing, shaping or queuing on the 1Gb download, but there's a chance the 30Mb
upload may choke overall flow behavior at higher utilization levels -- perhaps even well before download ever saturates. I would go to
dslreports.com/speedtest and report back your bufferbloat letter score. Although the "grade" is most relevant at full link saturation, it can still give you an indication of how your setup deals with higher levels of bandwidth contention. If it's much below a "B-" or so, you may want to think about playing with WAN queuing on the RV340, or potentially better still, putting in an
SQM-capable gateway. While SQM itself isn't a single silver bullet, and won't guarantee top performance for any one particular client or service, it will almost always deliver better results to the entire population at large, than if you didn't have it. Combine that will more traditional prioritization and shaping, per your own policies, and you should be able to iron out most quality issues, regardless of if you stay on cable or move to a new fiber service, irrespective of if you have one or two WANs.