Last night I completed the last stage (had to add some bits and take some bits off) of engineering my car. I'm glad I did it all and along the way I learned alot of things. My advice, don't listen to what your mate said or what you heard somewhere, call an engineer and organise a face to face to windscreen meeting.
I've spoken to nearly all of the engineers listed on the RTA site and alot of them you can't even get in touch with and when you do they can't be stuffed spending ten minutes answering your questions because they have 20 other guys ringing them each day!
Sorry Neil but that's not completely true. When they engineer something they include the specs so you can't do that and so that cops won't think that.
Like alot of you have said, cops can defect you regardless of a certificate but the benifits of having one is you can clear your car at the EPA just by providing the certificate.
If you don't have one, you have to clear your car by making it legal which could mean thousands of dollars getting that stock cooler on and changing the exhaust system and buying new wheels etc. So in the end a small outlay of $400 can save you lots of headaches and lots of money.
In a few months one of the clubs events will include an engineer and you guys can ask him whatever you wish.