i found out that the reason for driving with an unpainted kit is to let the kit settle in to place and work out the stress fractures that may occur. if you paint it with out doing this you may end up cracking your paint and then shell out more cash.
but then you have to worry about deep cleaning that kit to get out all the mud and little pebles stuck inside all of the unprepped pin-holes...
and ontop of that, since again you are dealing with unprepped fiberglass, it will soak up whatever crap it can... and will easily get stained. When the time comes to paint & prep it - you are left with having to sand-blast it to get a nice surface ready for primer and then paint & gelcoat.
if I was a bodyshop and you brought in a body kit to me that had been unpainted but installed onto a car who've driven around with it and now its all filthy, I wouldnt touch it - because I couldn't garentee that the final result would be of the best quality
thats my take on things... shoot me down if you believe me to be wrong