Using 1/4in strips, which are about 6.5mm, an 8mm staple would penetrate only 1.5mm into the form. The actual penetration would be even less if you left the crown of the staple sticking up to avoid damaging the wood and make removal easier.

I'm at work so I don't have a box of staples handy to check, but I'd guess that most builders are using about a 5/8in staple, which is about 16mm.