He already was a grown man who had peaked with the Revolutionaries and Hidden Mist. He was just slightly out of his prime and his glory days meaning probably not much stronger.
The real question is how much stronger the Sound 4 would've gotten. These kids were only 15 and still remain some of the strongest of their age group in the series. 15 year old Jirobo picked up something the size and weight of a mountain and threw it. Tayuya's jutsus were used effectively on Itachi and Sasuke. Everybody remembers how broken Kidomaru was in his time.
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