when i got my kit from viva, one of my side skirts were not fitting properly.. i just did some cutting and sanding and bending... it fit nice after all that and painting etc.,
it was some work to prep it and get it to fit nicely.. but it was very nice in the end.
i guess you will get better quality from the plastic kits.. but they cost shit loads more than the fibreglass stuff which usually does warp.