Actually, the customization is to match the threads, thread depth, fuel type, compression ratio, and plug seat. Your muffler won't affect those (except
maybe compression ratio). Assuming all your gassers are approx. the same compression, and use the same spark plug, you would only need one smart-plug.
Quoted from their website:
For experimental use only. All custom orders must include the following:
* Engine compression ratio
* Thread reach
* Type of plug seat (flat or tapered)
Maybe the price will drop (guessing $25-$35) once they are more mainstream