I've been reading a most excellent book Formula 1 Technology by Peter Wright, published 2001. He points out that volumetric efficiency hasn't increased much since the 1960s. Instead, the huge power increases have come almost solely from RPM increase. For example:

___1965: 120 bhp/liter @ 8,000 rpm
___2001: 275 bhp/liter @ 18,000 rpm

This indicates how revving any engine higher (even our A-series motors) will give more HP -- assuming the carbs, intake, valves and extractors can handle higher air flow.

Valve Spring Limitations
The first problem with increasing RPM was the valve springs. This was eliminated in 1986 by Renault's air spring system (pneumatic springs). Now the RPM limitation is not in the valvetrain, it's solely in the strength of the bottom end. Piston design is a top secret in F1 today.

The problem with high RPM
The problem with increasing RPM is that the resulting increased friction tends to cancel out any HP gains made from increased air flow. In other words, more air=more HP, but then that HP is used up in turning the crank (bearing friction) pumping air losses in the crankcase (below the pistons), etc. This problem was resolved in various ways including:

-- Pumping losses: The engine crankcase is divided into sealed compartments for pairs of cylinders. That's 5 compartments for the V-10 engines. The pairs help cancel out pumping losses.

-- Windage: The crankcase is run at high vaccuum by using scavenger pumps so there is no air for the rotating parts to stir up. That's up to 30 pumps per engine! This is an extension of the standard dry sump system used to prevent the crank from hitting drops of oil and wasting HP.

-- Friction: high tech coatings are used for all friction surfaces. Low friction oils are used. Parts are designed for minimal friction surfaces, for example, smallest bearings practical.

-- Accessories: careful design of water pump, oil pump, alternator, etc.

Notes on stroke vs. RPM
The piston stroke has dropped over the last 30 years (as we would expect) from about 70mm to 45mm to help with higher RPM, but ring speeds have still risen 30%. . The bore is increased to keep similar liter capacity (currently 3.0 liters). HOWEVER, a lot of the advantage gained is by making the engine block correspondingly shorter for increase Center of Gravity (handling advantages).