You don't ruin the standard system as such I mean yeah ok you have to cut it off but when you wanna go back to standard if you ever do its just a matter of getting the right clamps to put the old exhaust back on. You can get a mil eliminator for around £60-£70 but I've got sports cat so it's not triggered anyway. I'd say either go the whole whack and get sports cat or modify your standard cat so that you can put it in when it's due it's mot and then go back to decat for the rest of the time