As Chris says, its very much down to their experience, a few told me there was no way my Ings rear could be modified to fit as the carbon fibre would crack. This was having looked at the piece which isnt even carbon fibre
Had it fitted at the bodyshop that ESR use and it looks great.
Have heard that a lot of the cheaper or copy kits can be a real pain though. Watch for the "we had a problem so the price has gone up" crowd, there one near me that is famous for it. Quote cheap and charge the earth