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Almost all engines designed for rpm's above 1000 have valve overlap. I have modified cams on small Honda GX160 industrial engines that are designed to run at a constant 3600rpm. the problem they had was that with the govener removed for Kart racing, they would only rev to around 6000 - 6300rpm before they went flat on power. This was designed into the cam timing so that if the govener failed the rpm would be self limiting. After modifiing the cam I could rev to over 7500rpm and gain about 25% more power. Unfortunatly with the splash lubrication system they would go BANG after about 5 hours of operation when the conecting rod would brack due to the lack of lubrication at the crank shaft jurnal!
The basic problem is matching the cam timing to the engine speed to optemise the air/fuel flow through the engine. It is posible to design an engine that has very high rpm, large amounts of valve overlap and very good fuel economy. but it would only be good for a narrow range of rpm...
If you wanted to design a motorcycle engine for better fuel economy you would start with a long strock engine that did not need very high rpm to produce the power you needed, so that there is less rpm change with change in road speed, so that the engine stayed very close to its optimum rpm for FE most of the time...
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