There are two types of voltage that is important for the ignition system; input voltage from the battery side, and output voltage from the ignition coil. The input voltage affects the output voltage in most ignition systems, other than capacitor discharge ignition systems but there are limitations to the amount of output voltage that the ignition leads and the distributor cap can handle. Modern motor cars have a number of electronic devices, such as electronic fuel injection, electric fuel pumps, electronic cooling fans, etc. that reduces the amount of current that is available to the ignition system. For this reason, you may want to modify your ignition system by increasing the battery voltage, or by improving your car's wiring system.
Transistor High Energy Ignition (HEI) ignition systems draw quite a bit of current when the system first switches the primary, low-tension circuit to charge the coil. If the HEI ignition system does not get sufficient voltage feed from the battery, when it charges the coil, the system will misfire and you will lose power momentarily. For this reason you should ensure that no other electrical devices can draw current on the wiring from the ignition switch to the transistor HEI ignition system. You can do this by checking that no other device is connected to this part of the ignition circuit. You should also ensure that a minimum 10 gauge wire is used for this part of the ignition circuit.
You should do the same for points type ignition systems, but it is not necessary if you're using a capacitor discharge (CD) ignition system.
You can also improve the output voltage that is fed to the spark plugs by increasing the battery voltage. A general increase in the battery voltage will result in an increase in secondary coil energy, which will result in better spark quality overall, as well as at higher RPM. This works well on HEI ignition systems, but not as well on points-type or capacitor discharge (CD) ignition systems.
A HEI system with a 12.6 volt battery will provide good spark at up to 7,500 RPM on a V8 engine while a HEI system running on a 16.8 volt battery will provide good spark up to 8,800 RPM on the same engine! However, the HEI system must be designed to operate at higher voltages. Also, your other V8 electrical components, with the exception of the starter motor, are not designed to operate for too long above 13 volts. Fortunately, there are high voltage batteries with an additional 12 volt terminal on the market. These will allow you to run a high voltage HEI ignition system and your other electronic components on the 12 volt terminal.
Increasing the battery voltage on a CD ignition system should have no effect on the output voltage as a CD ignition system produces a constant spark regardless of whether the feed voltage is 10 volts, or 16 volts. If you do see in increase in spark quality when you increase the battery voltage, then you probably have a problem in your primary circuit. You either have a poor connection in your circuit, or the wire gauge is too thin to supply the required current.
You also cannot increase the battery voltage on a points-type ignition system as the higher output voltage will cause the contact points to burn and pit. The eventual result will be a weaker rather than a stronger spark, or no spark at all once the points are too badly burnt.