what you could do, is go and add any necessary noseweight now, then use some high density 2 part foam to lock it in place, killing 2 birds with one stone. That will protect it from imploding in flight. But the other problem that would be experienced at that velocity is aerodynamic heating. If the vehicle is going to be at Mach 2.5 for more than a few seconds, the nosecones friction with the air its going through is going to cause heat build up which could easily melt the plastic cone. In which case if would be advisable to install an aluminum tip to absorb the heat before it melts. But with all the trouble to go and do that, you might as well get a new fiberglass cone and use the plastic one for another project.
It all depends on how long it will be travelling that fast, 2-4 seconds and you would probably be alright, but i wouldn't try anyhing over that.
Do u guys think that putting a layer of Carbon fiber on the 3" PML Plastic nosecone
then putting a chemically inert clay type of stuff on the tip to absorb heat would work,
the stuff has a large Specific heat capacity so it conducts heat very slowly, plus it's very light.