This is my opinion, not a fact, so take it for what it is worth (current market value is about a penny).
Most modern cars have a strong ignition system because misfires and incomplete combustion cause a spike in emissions. Upgrading to aftermarket ignition components will often make a difference simply because it is done when stock components are worn. Now, I know that some of the earlier BMW COP modules showed less than stellar reliability (~80K mile failures) and if the aftermarket performance units have done something to improve the reliability, then they might be worth a look. However, from a guaranteed +XXhp and +Ympg standpoint, I don't think they are a wise investment.
One way that you can assess the strength of your ignition system is by plug gap. A hotter ignition system is capable of jumping a larger gap. Running too small a gap can lead to incomplete ignition (spark blowout) and too large a gap misfire. So, the OEs typically have the gap right. Compare your car to others and see where you are. My Fords run an .050" gap.
Also, I have a big problem with Bosch +4's. They shroud the spark too much. A modern ignition system should have ZERO problems grounding to a conventional spark plug under almost all conditions. If the +4s made a big difference over a conventional electrode plug (I used Bosch Silvers in my BMW) then it might indicate that the ignition system is not that strong.
Of course, if you want to get into you can hook up and oscilloscope and do an ignition waveform analysis.
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