FYI, the specific gravity of holly is .64. Mockernut hickory is .82 and osage is .86.
Here's the comment posted on The Wood Database about holly. "Comments: Holly is typically used only for ornamental and decorative purposes. It has a fairly large shrinkage rate, with a lot of seasonal movement in service, and its strength properties are mediocre for a hardwood."
I've worked with some holly but only for carving and inlay work per Pat's comments.
But what the heck, if you can get some well seasoned, it might be fun to try a bow out of it if you have some with pretty clear grain. Go for it.