Powerful as in estimated average penetration. Ballpark figures for a round from the GAU-8 at the middle of the engagement range is about 70mm RHA while an RPG-7 round is ~300mm. You’re comparing a 3 kg HEAT charge to a 1/3rd of a kg slug. It’s a different class of weaponry entirely.
The problem is that in a dive that steep, the time spent within cannon envelope is very brief. Guided weapons that can be lined up from a distance and fired from a much greater range are very different from rolling in, aiming, firing and pulling out in under two seconds. It’s one of those things that’s technically possible, but very much a hotshot maneuver of questionable wisdom.
Their size is the entire issue, though: it’s a 30mm cannon. It’s a very cool 30mm cannon, sure, but it’s still just an inert slugthrower. It’s a “tank buster” only in terms of the 1960s Soviet tanks it was made for, and there’s some debate about if even that wasn’t being overly optimistic. It services perfectly well at chewing up everything that’s not a main battle tank, which is the large majority of vehicle targets, but it’s not magic. The high rate of fire is the only thing that makes it threatening at all to a modern MBT, as it makes track and wheel damage likely from a side shot. Otherwise, it doesn’t matter how many times you shoot, if none of them breach the armor.
To put it in perspective, the round the M1 uses on other tanks is (at least) 25 times heavier and fired at half again the velocity, and anecdotal evidence from friendly fire in DS1 suggests that still wasn’t enough to penetrate another M1 from many angles.