Valve overlap info from the PDF:
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A given amount of overlap unfortunately tends to be ideal for only a portion of engine speed and load conditions. Generally, the torque at higher engine speeds and loads can benefit from increased overlap due to pressure waves in the exhaust manifold aiding the intake of fresh charge. Large amounts of overlap tend to result in poor emissions at lower speeds as fuel from the intake charge can flow directly into the exhaust. High overlap can also result in EGR which, although beneficial to part load economy, reduces full load torque and can cause poor combustion stability especially under low load conditions such as idle. Poor idle quality can therefore result from too much overlap.
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Overlap isn't good for low-speed operation. Not that you can increase it much without changing the camshaft anyway.
Valve Lift info from the PDF:
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Low values of valve peak lift will clearly restrict the ability of gas to flow into and out of the cylinder, effectively throttling the engine. Maximum engine power output will generally benefit from as much valve lift as possible up to the point where air flow becomes restricted by other features such as the manifold system or cylinder head porting.
It does not follow that engines should have the maximum possible valve lift as this can adversely affect low speed performance. Lower intake valve lifts result in higher gas velocity past the valve and this improves fuel mixing and combustion. For maximum torque at a given speed, lift should be kept as low as possible up to the point where the intake of fresh charge becomes restricted.
Part load economy will benefit from the intake being restricted by valve lift, as it reduces the need for throttling by the engine throttle and this will reduce intake pumping losses. Lift is not normally dictated by part load considerations, as this would severely limit the potential output of the engine.
The chosen value of peak valve lift is therefore a compromise between low speed and high speed full load requirements. A typical value for a production engine is in the range of 8-10mm.
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Reduced lift is good for economy.