Howdy
Of course, the answer you got from Uberti is correct, they are designed for SAAMI spec ammo.
However as with all mechanical devices, you will put less wear and tear on it with lighter recoiling ammo. If it was me, and I did not reload, I would go for the 200 grain stuff, not the 250 grain stuff. No top break revolver is as strong as a solid frame revolver. Every time you fire it the barrel latch and joint is stressed. Less stress will mean less pounding and less tendency to stretch the frame.
That's my take on it.
By the way, I don't own a modern made Top Break, but I own several originals. Yes, the modern steel is stronger, but the design and the principles involved are the same. I would go for less recoil and frame pounding.