I was going to spend money on the "Earth mod" after I measured 3.5 ohms resistance between the altenator case and -ve battery terminal today. However, I found that simply cleaning the battery terminals and cable clamps up the resistance dropped to 0.2 ohms

Car runs much more smoothly, and I got another 1 psi of boost this evening. (now holds 11psi constant) (Exhaustive testing required...)

Don't think the earth mod will be necessary now