OK...well, I don't think there's anything for me to say, aside of this one point:
If a manufacturer gives a specification, there's probably a reason for it. Yes, you probably can jam 89 hp worth of modules into each row of a Rackbrute. You might NOT want to do that, however, and I think I know why Arturia's spec is what it is: heat.
I learned the basics of synthesis on an ARP 2600, way back when. And one thing I'd always found odd back then was all of the air-gap between the synth and its case...at least, at first. But the first time I ever had to calibrate it, everything snapped into place...because analog circuits are VERY susceptible to problems resulting from heat buildup. Interestingly, every iteration of the ARP 2600 that I've used, from rev.2 thru rev.4, and even on Behringer's 2600, has SOME sort of ventilation to keep the circuitry stable.
When you're dealing with power components, these generate quite a bit of heat simply because of what they're being asked to do. And that heat can build up inside the case, causing instabilities (like, say, VCOs that don't stay in tune) that are annoying. But if it were just a case of making a synth work in a wonky manner, I wouldn't sweat it (much). But it's not.
Heat also causes damage over time, especially with electronic components that are generating heat. Those components require some level of ventilation, otherwise the heat buildup within the case...as well as the on/off cycling that takes the synth from room temp to whatever it would be under load and back down again...will take literal years off of the lifespan of the gear. Yeah, 1 hp doesn't sound like much, but if you put that entire 1 hp "in" as a gap along the left end, where the P/S is, this WILL provide quite a bit of ventilation for the supply module. The lower row provides an "inlet" for cool air, and the hot air will simply rise out of the 1 hp slot next to the module. And yes, I know that some people argue that the panels in a modular provide some heat-sink-type behavior...and they do, which is why the lower-temp modules use that to dissipate their heat loads to some degree. But the major heat source is the power components...which also includes heat generated by current flow in the busboards and DC rail wiring, and that has to get out in some other way.
Plus, if your power module pops (which could easily result in a catastrophic failure in some P/S designs...as in modular go "pop!") and Arturia finds that you had 89 hp per row while they specced 88 instead, they could argue that you've voided your warranty, provided this were to happen while the cab was still covered. Those guys know what they're doing...and that's why I pay attention when I'm dealing with any sort of sizable heat source that's part of a musical device.