If the A.I can self-improve, then we can also improve it. We're still equivalent.
Hitler had literally millions of human-equivalent-intelligences - they even had excellent sensor and actuator suites - and it didn't work out so well for him in any case.
Sure, you're right in that humans don't understand enough about our own brains to perform such modifications, but that may simply be down to a lack of knowledge, rather than not having sufficient cognitive ability to comprehend the brain.
Additionally the hardware which an AI can run on is generally assumed to be significantly more malleable than our limited gray matter confined within our cranium
The problem is there are hard physical limits you run into in computation - it's more efficient to just do the chemistry than it is to simulate it. Our meat brains might be more efficient in terms of density and power efficiency than any physically feasible digital computer.
Further, computing power can't be increased arbitrarily, especially when networks and switching come into play. Even if your simulated brain worked out how to improve itself, the resource cost to actually do so might be prohibitively high.