Matra used a very thin autoclaved glassfibre win my experience hich remains quite flexible and is even said to change shape on a hot day.. Repairs are generally of rigid glassfibre which will break away from the broken area.
I think of Hans' experienced as documented in this thread
http://www.matrasport.dk/forum/index.php?topic=167.0A repair will inevitably add some material and therefore be a bit more rigid, but it should never break away if the damaged parts have been cut away and the remains sanded and cleaned well so the epoxy can bind correctly to the old part.
But of course - and probably the best since this is probably also an insurance case - it will always be better to replace with a new panel.
- Anders