No, he wouldn't. Morthoron's point seems to be that sheer fighting prowess, however great, is not enough when you're facing a Balrog; you've got to have an extra something in you that only those have who have seen the Light. Which may well be true - and anyway, it's a beautfiful idea.
It's not so much that your having been to Aman would affect the Balrog in any way, but the way it affects
yourself - makes you less easily daunted by evil and terror, strengthens your courage and your trust in Eru, something along these lines. And all this, of course, would be true in any other fight as well.
So maybe the poor Dwarves never had a chance. But wait a sec... What if Gimli returned from Valinor and used a time machine?
