Hi Steve,
The honest answer is I don't know!
I fitted that bumper this way a couple of years ago and don't remember it causing me any hassle so I presume I fitted it with the original screws.
If you're going to fit it to the front of a plastic chassis then you're going to have to cut it down to go around those raised sections too but that should be easy enough.
A screw is a screw and mine has a collection of various M3 screws from various sources nowadays as it's been apart many, many times. I've found the FTX screws to be a bit rubbish and the hex easily rounds off so I'd rather replace them all with something else if I had the choice. (If you do manage to round one off then it's possible to tap a appropriately sized torx/spline into the hole to get the screw out.)