Unfortunately there appears to be nothing on the market now that protects from stone impact. Stonechip seems to be favored, but it isnt designed to stop the stone piercing it and causing the damage as it occurs with the shock wave that passes through the fibreglass.
The only solution I found was to get some 2mm rubber sheet and bond it to the underside of the wheel arch. It isnt a cheap solution as it will set you back around £60 to do all four arches. However it is good value when you consider the cost of repair and respraying.
I got my rubber sheet from Grommets - see
http://www.grommets.co.uk/extshe.html
Dead easy as you can cut it into squares and apply like the space shuttle tiles. The finish is gret also. You apply using evo-stick or similar contact adhesive and just give the rubber a quick wipe with white spirit before applying. When done you can see immediately that it will do the job and rest in peace.
Mike