How Much Attic Insulation? (R-value vs Ceiling Bowing)
5/8" drywall on 24-inch centers is standard practice across the industry. Unless you have some weird early-version panels with poor sag resistance, 15" of cellulose will not cause a problem. 

I'm with Daniel Griscom, however. R-49 is in the ballpark of what we install in new homes here in frigid Minnesota (regularly seeing -30F in the winter). It's almost comically excessive in your case. You should look for something in the R-20 to R-30 range as your sweet spot (bang for the buck, ideal payoff, whatever). If you're feeling chilly when it gets way down in the 60s like it does there, go nuts. Just don't expect to see the value in your "heating" bill. :)