on a p-51...strip flaperons
you may gain some benifit?
your more likely to cause more tip stalling by dropping those ail down.
most modern "sport" p-51 have a lite enough wing loading.
PLUS this ups the posiblity of pluging them in wrong and ending up backwards.
You'll notice that the aerobats don't do flaperons.

now a trainer has more stability. But even then, the benifits of flaperons do do much more.

I wouldn't do it. save flaps for a plane that has them seperate.