Well, I did a random sampling of X-boat routes off the Traveller map. I'd say there's no need for a J5 or 6 boat whatsoever, and that most X-boats should be J3 with a smaller portion J4.
At this point I have an extremely important question about your methodology for reaching your conclusion.
Were you just simply measuring the distance between sequential systems along the marked routes?
Or were you counting all of the systems within Jump-4 range from each point?
For example ... using the Spinward Marches sector, although there isn't an explicit XBoat "line" drawn between Rhylanor/Rhylanor and Risek/Rhylanor, the Risek system is in fact 4 parsecs away from Rhylanor. So the path that an XBoat would take from Rhylanor to Risek would be a direct Jump-4 ... not a Jump-2 to Jae Tellona followed by another Jump-3 to Risek so as to "follow" the lines drawn on the map.
Another example ... XBoats relaying communications from Aramis/Aramis to Rhylanor/Rhylanor would only make a pair of 4 parsec jumps. The first 4 parsec jump would be from Aramis to Celepina, followed by a second 4 parsec jump from Celepina to Rhylanor. The lines on the map would make it look like the journey would be Aramis to L'oeul d'Dieu to Celepina to Jae Tellona to Rhylanor (4 jumps) but the actual XBoat traffic from Aramis to Rhylanor would instead be only 2 jumps of 4 parsecs each (as described).
There are a remarkable number of these locations along the XBoat Network where the pathing of the lines of the network might imply that the jumps are shorter and thus more jumps are needed to move communications along the network, but that's just an artifact of how the lines get drawn on the sector maps. The way the network actually functions is that XBoats departing a system can jump to ANY other system ON the network within 4 parsecs of the system being departed from, regardless of how the lines are drawn on the map. An example of this would be an XBoat being sent from Ghandi/Lanth to Denotam/Vilis, 4 parsecs away, even though there is no "directly line" drawn on the map between the two systems (because it would get way too confusing way too fast if they were drawn everywhere). Both systems are on the XBoat network and are 4 parsecs apart, so an XBoat can jump from Ghandi directly to Denotam without needing to "Klingon Hook" around Dinomn and Extolay (following the lines on the sector map) before being forwarded on to Denotam.
As you can quickly see, depending on how you "count" the transit opportunities, a lot more of the network can wind up operating at Jump-4 than it might as first appear.
Even if the Express Network was not expanded out to new systems, there are some unique opportunities on the existing network for Jump-5 (and of course, Jump-6) upgrades. Jump-5 would connect Regina/Regina to Inthe/Regina, dramatically reducing the need to "take the long way around" through the Lanth subsector between Rhylanor and Regina. It would cut transit times from Rhylanor to Regina down to 3 jumps through Inthe, rather than the current 7 jumps going through the Lanth subsector. Another example would be a Jump-5 shortcut from D'Ganzio/Lanth to Lunion/Lunion in a single jump, rather than the 4 jumps needed going around through Fosey/Mora when limited to Jump-4. Needless to say, just in the Spinward Marches alone there are numerous examples where simply making Jump-5 available has some pretty profound effects on "unclogging" the transmission of communications around the network caused by the less efficient arrangement of network route locations.
And of course, Jump-6 provides the highest efficiency of movement along the "backbone" of the network for subsequent dissemination using the legacy Jump-4 ships away from the high speed network backbone. So instead of having a hi/lo mix of XBoats (jump-4) and Scout/Couriers (Jump-2) to propagate communications along the network, instead you wind up with a hi/mid/lo mix instead of J6 XBoats, J4 XBoats and Scout/Couriers all operating together to provide the necessary "packet switching" around the network at maximum speeds through jump.
Blah blah blah, you know the drill.
