Just a few thoughts on this, bloke at work had his plates nicked on a focus a few weeks ago, just unscrewed em and left his car bare. He rang the police to report the theft who told him not to drive anywhere without them. He was at work and you need logbook to get new plates, then the speed camera letters start to land on his doormat...
I decided to fix mine, first i got a roll of no more nails tape, under a £5 from Tesco / B&Q or similar, tested in auto express it took over 20KG of force to remove a numberplate secured with it - the plate will break up long before this and at least stop someone else nicking it.
I then replaced the philips screws with some Torx T20 I had, the chances of your local scumbag having a Torx driver to hand have gotta be low.
Just give it a go - £5 got to better than dealing with the hassle if they are stolen. Being a sad tw4t I painted the torx screws to blend in, 1 white, 1 yellow and 2 black - right through letters !!