Its more than a 1/3, at 14.7 psi (at sea level, less as you go higher) youre using DOUBLE the air. Banks had a similar claim, stating that going from the supplied 3" to the "monster" 3.5" exhaust netted about 5hp at 3300 rpm. When i was running my 088, it had 3" with a glasspack (crush formed), when i replaced it with 4" mandrel from the down pipe back, i gained like 4-5 pounds of boost at WOT (could hit 18), egts were cooler at WOT as well. This was an 088, with a massive leak in the drive side, and a maxed out 275k pump.

All that said, the fuel specs that ATS and Banks recommended were hardly anything above NA, in that context, the larger exhaust probably wasnt necessary, but 10psi vs 20psi is a substantial difference in air flow, potential fuel burnt, and thus creating exponentially more exhaust.

That's because it *is* an RPM governor... though the governor response is 'tuned' a bit low, so it droops under load more and is thus a 'road governor'.
If the pedal controlled the rack, you wouldn't have a governed speed really - it would go much higher until it physically couldn't push enough fuel to maintain that speed.
As it is now, unloaded peak RPM isn't much higher than loaded peak RPM, because governor.

would be nice to know what the power gain is at normal rpm, and if it's a power gain or also an efficiency gain.. a few hp at 2000 rpm gained without extra fuel burned would be quite a loss freed up. I'm guessing the actual hp required to push one of these trucks down the highway is probably 40 to 60 hp, a few hp freed up would be a small but noticeable mileage gain.

Back in the day, about 1990-91, there was nothing Banks DIDN'T make a claim about. They had a high dollar non-turbo air cleaner element that we just had to have according to their adds. I phoned their sales pitch line one day and asked how their A/C element breathed better if my Filter Minder wouldn't move when towing a max loaded gooseneck up a steep grade with my stock air cleaner? The sales pimp's answer was downright hilarious. They also claimed their turbo system put out WAY more power than ATS. They failed to mention Banks used a sea level 180 HP 7.3 engine whereas ATS in Murray, Utah used the high altitude 7.3 IDI version that was 160 HP for test results. And when I was researching to death which turbo system to buy, Banks people had nothing positive to say about their own, they just badmouthed ATS up one side and down the other. That's always told me a lot about which product to buy. Especially after Ford chose ATS in '93.

That sounds like a failing on ATS's part. Most IDIs were the 180HP version... and, if you crank the fuel up(as Banks suggests), that calibration is lost anyway.
I can't say about the sales info, all I know is that the (wastegated) Banks Sidewinder is, hands down, the best "kit" turbo I have seen run. It spools faster and tops out similarly to the ATS, pushes more boost.
Also, the turbo placement is way better than the ATS - Getting to the #5 and #7 glow plugs with a ATS 093/Factory turbo kit is a real pain.
Considering ATS let Ford butcher the factory kit they used, I'm not sure that's a good reasoning. Now, I understand it - Ford wasn't going to modify the firewall to get clearance for extra piping, but still, I could totally see that being a deal-breaker for Banks.

As a disclaimer, I currently have: 2 Banks Sidewinders(my dad has a 3rd), a Ford Factory kit/093(second kit waiting for a rig), and an ATS 088 kit.
(also my friend has a Hypermax kit on a non-running truck; no clue yet how that performs).