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I'm assuming that the specs you're listing for the Prius are for the intake cam and not for the exhaust cam. The long duration is needed because it is an Atkinson cycle engine. The intake valves stay open during part of the compression stroke, effectively creating a smaller, lower dynamic compression ratio engine with a high expansion ratio for improved thermal efficiency. That's also why the Prius can run 13:1 static compression ratio on 87 octane gas.
I will say though, that the cam ramp angles are limited by the weight of the valves and stiffness of the springs. At some point, the improved efficiency from reduced pumping losses will be outweighed by the increased friction caused by the need for stiffer valve springs.
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