Perhaps I can shed a light on this subject. The fuel system is principally a balance between orifice size and pressure.

The engine-driven fuel pump has aneroid which puts upper deck air pressure to regulate a needle valve. If pressure / flow adjustments on this needle valve are made without a fuel pump on, to add engine-driven fuel pump pressure will increase flow. (Hi Side)

On the TSIO 550, above 10,000 feet- use of  low pump is a relatively small increase. Principally it is restoring the fuel to the inlet side of the pump. But on take off, you probably couldn't tell the difference.

Have you ever noticed that you cannot shut the engine down with the mixture control if low pump is still on?

On the TSIO 550, the use of high boost at Max power will increase the fuel flow 3 to 5 gallons an hour. At some point the mixture will be so rich the engine may quit. It definitely will be smoking.

If you set up your engine fuel system adjustments to meet the design specifications, you could do it either pump on or pump off. With the Lancair, Continental is recommending pump off. But you should know that if you alter the pump use procedure, you will not be meeting the design specifications.  If the settings were made in the opposite configuration.

Some aircraft manufacturers use low pump for takeoff and landing, but we do not advocated in the Lancair IV because we make our adjustments with pump off.

If the fuel pressure/flow is too high on takeoff, you can pull mixture until fuel flow is 43gph and the engine will operate normally. Much above that and it will begin to smoke. This would indicate it's time to do the fuel adjustments over and lean the high side. Leaner than 41gph and detonation is a possibility. The engine can be destroyed in less than a minute. Check FF on every TO.

The low side pressure adjustments should be 7 to 9psi with electric pump off at 600rpm.

Then idle mixture set to 50 rpm rich. 

In that order.

 As an aside, Continental likes to call the primer a "cold start option". The best prime for these engines is to put fuel into the intake valve area through the injector nozzles. (i.e. mixture). But in the extremely cold conditions, the primer will definitely aid in getting the engine running.

Also using the high pump is a excellent aid for hot start by circulating fuel from the relatively cool fuel tank into the fuel pump and back out the vapor return line with a mixture and throttle in the cut off position. 

That will not take care of hot fuel in the lines from the engine-driven fuel pump to the nozzles. Usually low pump after the engine start will drive air in those lines out. Or the primer works also. 

Regarding the use of high pump in-flight as an emergency procedure, you can always use high pump pressure in case you were to shear a engine-driven fuel pump drive. If it's too rich -pull the mixture back.

IMHO this is a great system that has a lot of redundancy.

Supplied clean fuel /set up/ and operated properly.

 It'll get you home.