i know it's tempting, but there's nothing like a buddy of yours seeing your car like any other day, and the next time he sees it, it's a completely different car. my vote is to wait and get it all done together.
Don't put it on there unpainted......your car looks good as it is, and then you won't have to sit around with mismatched body pieces until it is time for paint......no spoilers look sweet IMO.