Well, today I needed a 50 for those plie squats. All I had was a 40#. Now I opted not to buy plate mates or pace weights, but instead went to Home Depot and bought a bunch of heavy duty magnets for a few bucks. I attached 2 of my 5# barbell plates to the ends of my 40 to make a 50, BUT it got awkward. I think the 50# DB would be much better.
I would say YES -- if you truly going to do STS in the manner intended, you WILL gain significant strength and will likely use these larger bells. Good luck.