For my 2 cents worth, I'd have to agree with 427Sharpe and Cobra 20646. If the battery smoothly turned the engine over for 5 minutes (or even two minutes, for that matter), the problem was not the battery. The battery supplies the power necessary to turn the engine over, and also the power for operating the ignition until the alternator is spinning at sufficient speed to supply the required current at the proper voltage. (That is why the alternator is designed to generate 14 to 14.4 volts while the battery is usually around 12 to 12.4; the electrical power for normal operation will then be supplied by the higher voltage alternator rather than the battery.) Most starters are capable of spinning the engine at approximately 250-300 rpm maximum. At this engine speed, the alternator is normally not generating voltage equal to that being supplied by the battery, so the battery is still providing power to the ignition until the engine starts and comes up to at least idle speed. However, if I remember correctly, the MSD system should be capable of operating at voltage levels down to about 9 volts. My MSD 7AL system has started my engine when the old battery was so low the starter would stop momentarily between engine revolutions. If your battery is spinning the engine correctly and you still have 11.5 volts at the terminal during the starter operations, the problem is most likely not the battery.
The 12-volt automotive battery itself is made of six, 2.0 to 2.1 volt cells, connected in series. There is some small voltage drop across the connectors, so the total voltage across the terminals should work out to (6 X 2.1) minus the cell connection voltage drop to equal the 12 to 12.4 volts commonly seen at the terminals. Also, most automotive batteries are designed for high-amperage, short duration power surges. If you look at your Optima literature, or go to their site, they state that the starting batteries are designed for putting out maximum amperage for 5-second durations, as is typical during starting sequences. Most automotive batteries only use about 3-5% of their capacity during a starting sequence. They are not really designed for "deep cycle" discharging such as is seen in batteries used for golf carts and trolling motors; that's why battery makers produce "deep cycle" batteries with different dimension to the internal lead plates. Deeply discharging a starting battery will usually have some small penalty in battery life expectancy. That is not to say it will suddenly fail (although, like any battery, it may for a variety of reasons) but rather you may have to replace the battery a month sooner than you would have had to otherwise.
If I may make one suggestion: Check the MSD website
www.msdignitions.com and look under their troubleshooting guide. They have some quick checks to go through to determine if their system is bad. If you still suspect that the ignition is ka-ka, get in touch with them via e-mail. I have personally worked with them on problems with one of their components and found them to be extremely helpful and professional, and they corrected my problem in a fair and responsible manner. Good luck.
Let's not be hasty here...
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