Will simply replacing the stock Toyota ignition coil with an after-market unit (without changing anything else) improve the power of the spark? A performance shop told me "no" since the coil is downstream of what actually creates the power. However, I have read other articles where that is not true.

 

The very slight gain you might see isn't worth the cost. Even SC'd Runners don't need an ignition upgrade since the OEM system is quite good on Toyotas;I've never even heard of people needing to upgrade the ignitions on fairly heavily modded turbo Supras.