There's an unfortunate misconception that Justinian depleted the treasury and squandered the empire's forces in vain. When Sicily surrendered, he gained control of a rich and fertile island. He quickly conquered other vital and strategic lands across the Mediterranean, marking the zenith of the Empire.
From today's perspective it looks dumb since the bulk of the areas weren't in Byzantine possession for long, but you have to look at it from a 6th-century perspective. No one knew how or when the Lombards would storm Italy, for instance, or when the Arabs would overrun his Egyptian and Syrian provinces. And North Africa continued to be well-tilled and valuable until it was lost to the Arabs in 697, almost 150 years after Byzantine generals conquered it.
At the most, it changed the makeup of Europe and established Byzantium as the most powerful state in the world for nearly half a century.
I also think that the Byzantines should be on steroids around 963-1025, but to a lesser degree.