This has been a hot topic since the days of single versus dual point versus magneto ignitions and a lot of bad information out there from people who have spent a boatload of money on aftermarket ignition components that make zero difference but they are too embarrassed to admit such afterwords.

I have run a lot of stock GM HEI as well as those with an MSD coil and found little difference in most street situations. The stock HEI lets you comfortably run a . 045-.048 gap and the MSD coil is good for .050-.055 which gives a lot of real hot spark and clean burn. Both combinations perform well up to 6,000 which is well below most driving conditions most of the time.

If you live in a 6,000 -7,000 RPM world on a regular basis things change and those who tinker for .01 improvements in an ET will build some serious ignition systems that are designed to give you a big blast for the few seconds you need in a 1/4 mile.

Read these two articles before you invest any money:
Ignition Performance Test - Hot Rod Network
HEI Distributors - Addressing 10 Common Myths - Super Chevy Magazine

You may find it best to stay stock or perhaps a MSD coil.

Glenn