The important difference between low- and high-impedance injectors is the amount of electric current they require.  The high-impedance injectors used by carmakers do not use much current.  In fact, even if you connect one directly to your car's battery, it will not draw much juice.  This is good for the car maker because the circuit in the ECU that runs the injectors can be a simple switch made from cheap, light-duty components.

A low-impedance injector needs a bunch more electric current to open, about 4 times as much as the high-impedance injector.  This is so much current that the injector will burn itself out unless the juice is somehow turned down right after the injector opens.  Now, instead of a simple, cheap on/off switch, the ECU must use heavy-duty components that can handle 4 times as much current.  The switch must also turn down the flow of electricity after the injector opens so the injector doesn't burn up.  This is a much more expensive circuit, and one that generates a bunch more heat inside the computer.   Getting rid of that heat raises the computer cost even more.

This business of the ECU yanking the injector open with full current, then turning down the juice to keep it open, is why you hear low impedance injectors called "peak and hold".  It takes a big current (the peak) to yank them open, but a much smaller current (the hold) to keep them open.

So why bother with low-impedance injectors if they're harder to run?  Because bigger electric currents can yank harder than smaller ones.  As the flow rating of an injector increases, so does the weight of whatever the injector uses to turn the fuel on and off (for example, a pintle, or a disk).  This is no big deal for stock injectors in the 18 lb/hr to the 30 lb/hr range.  Even though they are high-impedance, they're light enough that the small current can yank them open quickly.

But big (like 42 lb/hr and up) high impedance injectors are slow to open.  This makes it hard to get a good idle, and kills throttle response.  What if you need an injector larger than 42 lb/hr?  Simple!   You buy low-impedance injectors, which are available up to 160 lb/hr.  The large peak current can quickly snap these monster injectors open.

So a low-impedance 50 lb/hr injector will almost certainly be "snappier" than a high-impedance 50 lb/hr injector.  When the 50 lb/hr high-impedance injectors on the Acceleronics test car were replaced with 75 lb/hr low-impedance injectors, the ECU setting that adjusts for "sluggishness" had to be set back to stock.  The high-impedance 50 lb/hr injectors required much more time to open than stockers, but the low-impedance 75 lb/hr injectors are just as snappy as stockers.