Doesn't sound like a bad temporary solution, so upvoted.

Sadly adding spash damage doesn't fix the underlying issue the miniguns have and creates the problem that you reported of being unrealistically powerful against buildings.
BI has to make them less accurate and have a realistic rate of fire: from what I could test, the miniguns on the AH-9 have a _combined_ rate of fire of around 1400rpm and with good FPS (50ish).
That means that each minigun has a ROF of around 700rpm, which is nowhere near the minimum of 2000 that those thing can reach, and even farther than the maximum rate of 6000rpm, and that's when the game doesn't slow down, reducing the ROF we already have.

Still an issue: the AH-9 miniguns have a ROF of around 700rpm each (for a total of around 1400rpm), while the UH80 and Orca have a ROF that is closer to the minimum of 2000, but still below it and are still affected by the actual game performance.