I always say to such tests that they're unfair, you're using one gun that has wildly more DPS but also uses much more flux, and the other gun both uses much less flux as OP. No wonder the conclusion will be "this things kills quicker". It's like testing the Assault Chaingun versus a Heavy Mauler.
Determining effective DPS versus any one ship's armor is the first step in making meaningful comparisons in terms of flux expenditure required to kill X. It's definitely higher for the HAG. I'm curious as to what I get for that expenditure, and where the break-even point lies in terms of armor strength/hull strength. Maybe I should bring torpedoes instead of sabots when using HAG, I'd love to know!
Technically speaking, the Hellbore is always going to be a pretty efficient weapon due to how armor mechanics work. Outside of the Invictus, only a few up-armored Onslaughts/Legions can take more than 3 Hellbore shots on one armor cell without being crushed (without skills). That means that 2250 flux can strip virtually any ship in the game. Once you get through armor, that 750 damage round is going to have high hit strength against the meager 5% residual armor. At best, you'll mitigate maybe 10% of that hit, more likely in the lower 5-6% if you're a Cruiser. So, just from an efficiency perspective, the Hellbore is going to be more so than any HE weapon that has a 1:1 damage/flux ratio because of its huge damage/shot. The HAG is 1:1 so by virtue of it having a much, much lower damage/shot, regular armor and residual armor will mitigate its damage considerably more.
But, that doesn't tell the whole story. The Hellbore has a massive accuracy issue and can be shield flickered pretty easily. HAG doesn't have those issues and can keep steady HE pressure on everything from Capitals to Frigates. You will pay more flux for the same damage vs. the Hellbore but if the Hellbore misses (-100% efficiency) or hits a shield (50% efficiency), you're out both flux and time. Plus, the raw DPS of the HAG is nearly twice as high. Getting through hull is arguably the more important consideration and raw DPS plays a bigger role in that.
They're just two different weapons on opposite sides of the HE spectrum. Personally, I think the HAG is a fine weapon, albeit a little less general-purpose than the Mk. IX due to the lower flux cost. It's just not very flashy.
Edit: Just did some brief testing and I guess I was mistaken about the damage calculation vs. residual armor. When testing the Hellbore against a 900 armor sim Enforcer, it was doing around 725 damage per shot against "naked" hull. That didn't make sense to me until I realized the armor calculation does factor in the HE multiplier, but only in the second half of the equation. So it's 750*(1500/(1500+45)) which works out to be 728 damage. So, back to efficiency, that's 97% efficient against Cruiser-grade armor. The HAG can only boast 84% efficiency against the same hull. It also means Kinetics get their damage halved for the armor portion of the calculation. A Mk. IX did ~540 damage from 4 rounds, which tracks with the Kinetic portion of the damage being reduced to 68% efficiency by a paltry 45 residual armor.