Ive been out an had a look at the zip-ties and they seem fine. I was out in the car for a couple of hours last nite on, lets just say, not exactly the smoothest roads. They were really bumpy where pieces had been replaced etc.
I dont think they will loosen up because of the way they are. If they had been the opposite way round,the square,rachet type end up by the exhaust mount i think it would loosen but because the square bit is at the bottom i think it will be fine!
The cable ties are still as tight as they were on friday when the car came out of the garage after they had been done.
Could you slightly melt the zip-tie by the bit it end goes through to stop it moving?