Why in the Seneca 2 the Rated Horsepower are higher at 12000 feet than at sea level? (215Hp at 12000feet, 200Hp at sea level)
Back pressure affects power because the more pressure opposes the exhaust, the more exhaust gas is left in the cylinder when the exhaust valve closes.  This trapped exhaust gas, effectively inert for purposes of the engine cycle, limits the amount of intake charge that can be drawn from the manifold at a given manifold pressure.

The more intake charge you have, the more power you get (within limits), so the less unscavenged exhaust you have diluting the intake charge, the more power you can generate for a given manifold pressure.

This effect can be mitigated to some extent with valve overlap -- where the exhaust valve remains open for a short time after the intake valve opens; this allows the incoming intake charge to push some additional gas out through the exhaust valve -- providing there isn't too much "back pressure" on the exhaust side.