I would think that old steel World Sport would soak up the bumps fairly well. Not sure of the current tire size on the bike, but larger tires work well on bumpy surfaces.
I’m guessing that you will get a little improvement just from better hubs and lighter weight, especially if the current hubs are pitted.
You might get a little gain from better gearing with closer gaps or lower ratios if you have climbing on your routes.
I agree that speed increases with a new bike may not be earth shattering. If you are able to ride in the drops, you might pick up 1/2mph with no more effort.
That said it is probably never a bad decision to upgrade from a lower level heavy 35 year old bike if you are looking for more performance.
John