Impeller Nodes pearl 1
The impeller rings of any military starship mount a total of 24 nodes: 8 alpha nodes and 16 beta nodes. The alpha nodes are mounted at 0, 45, 90, 135, 180, 225, 270, and 315 degrees. The beta nodes are mounted every 15 degrees between the alpha nodes. In terms of size, an alpha node is about three times as large and massive as a beta node; in terms of the generator support required, the difference is more like six times as great, but a beta node provides about half as much power to a standard impeller wedge as an alpha node does. Thus each alpha node provides about 6.25% of a wedge's full power and each beta node provides about 3.13%, so that the alphas and betas as groups each provide 50% of the whole. (Actually, those values are halved for the full power of the wedge, since both impeller rings combine in a full-strength wedge.) The 8 alpha nodes, however, suck up as much mass as 48 beta nodes would, which is the reason beta nodes are used. It is the alpha nodes which contain the Warshawski sail components, though, and a ship cannot generate a Warshawski sail without at least 8 of them. It is possible to run a node at greater than 100% of rated capacity under emergency conditions in order to get back some of the power lost when other nodes are knocked out by combat damage, but this is a risky procedure and not one to be undertaken lightly. Freighters may sometimes carry fewer beta nodes, or even none at all. It is extremely uncommon for a ship to mount no beta nodes, but it is not unheard of, either, since cargo carriers seldom carry the inertial compensators to permit them to make full use of a "full powered" wedge, anyway.
And
Beta nodes pearl 2
Impeller rooms are cylindrical volumes located within the hull and centered fore-and-aft on the impeller ring they serve. The diameters of civilian impeller rooms are approximately 60% of the diameter of the impeller ring and approximately 1.2 times as long as they are across. They normally consist of a single, very large compartment crammed with the required generators and node support hardware. Military impeller rooms are approximately 85% as wide as the diameter of the impeller ring and approximately 1.6 times as long as they are broad.
So most of the reason a military impeller room is big, is actually due to the armoring, and all the beta nodes. Freighters on the other hand, primarily have just 8 Alpha's, and maybe a couple beta nodes to boost their in-space speed slightly.
There's no real need for maintenance on just that, and as far as I understand it, the real cause of merchies being "slow" is their compensators and hyper generators themselves. There's not much (if any) size differential in the hyper generators of military and merchies. Which places most of the speed, or lack thereof, in the compensator.
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On the economic side, trade in the honorverse really doesn't seem to matter who does the freighting it seems. Flip a coin, whatever you get is the shipper you use, seems to be the general concept. Hauptman Cartels, Dempsey Cartel, or any of the other known Manticoran "Trade Lords", all appear to be the same.
The reason someone like Hauptman Cartel (or Grayson Skydomes) are more successful, is the spotting of opportunities, and seizing them with both hands. Honor saw the potential in the dome-tech, and found a way to change the rules to allow her to invest into it, when she couldn't do it as just a steadholder. And as I pointed out in OP, Hauptman has been known to... creatively upgrade ships (the Atlas liners), or building the Nat Turners which had a high level of tech, compared to any other navy in existance.
The only place I can possible see an economic reason to pick certain shippers, is prior to Laocoon, shipping duties on using the Manticoran Junction may have inspired Solarian shippers to use Manticoran hulls for lower Junction fees. As duties went up and up, leading upto the Havenite Wars, it would have become more cost efficient to use Manty freighters to get around Junction fees, and then possibly switch back to Solarian hulls after.
That opinion is based on the Mandarin's description of exactly how much freight in the Solarian League moved in Manty hulls, for X% of the trip. Off-hand I think it was something around 50% of total trade moves at least 75% of their travel distance in a Manticoran hull.
Excluding that thought, one shipper appears to be the same as any other shipper. So after the MAlign is taken care of, and other sectors copy the Maya Sepoy Option, a shrewd merchant (like Klaus Hauptman) would be looking for new opportunities to exploit and increase their bottom line profits. Wouldn't that fall under sometimes requiring short-term expense hikes, for long-term profit margins to increase?