Actually, the power output of a normally aspirated engine will initially increase during climb from sea level. This is because the back pressure on the exhaust side decreases, and that effect overcompensates the power loss due to decreasing air density.
As queried by IO540, I've also never seen any data to support that contention and, assuming the climb is undertaken at Wide Open Throttle, it seems highly improbable.
I very much doubt this. Exhaust back pressure on aircraft is quite minimal compared to car engines etc, and even modifying the exhaust on a n/a engine for more power still yields very small gains. Higher alitude, less air, = leaner mixtures = less power. Exhaust plays a minimal role.
But I don't believe that to be correct either! For a constant MP (together with constant RPM & peak-power mixture), altitude makes a significant difference to power output. Whilst this is actually the impact of lower temperature as well as reduced exhaust back pressure, it's quite wrong to suggest that the exhaust element contribution is minimal.
Some hard data for the power generated by a Continental IO-550 at 21"MP and 2100 RPM on an ISA day: sea level = 146 BHP, 8500 ft = 169 BHP. That's approx 2% increase per 1000 ft.