He wouldn't be that strong, I think that he would be around East Blue's kingpin level (Krieg/Arlong, in terms of strength/power at least, but with his wits he could get much higher). In order to steal abilities, he must meet 4 conditions, including having the victim touch the handprint of his book. Most DF users of One Piece have superhuman strength, and that superhuman strength usually far surpasses HxH's superhuman strength (HxH's strongest humans can maybe push the 256 tons Zoldyck's testing gate... OP's strongest can destroy mountains with punches and islands with DF powers).
Most HxH's top dogs can be severely hurt or even killed by bombs (e.g.: Hisoka), Mr. 5 would be a huge threat already. Even if we give Chrollo Uvogin's toughness (probably the toughest HxH human seen so far, as he was able to stop anti-tank missiles), I doubt that he would be able to withstand a Gura Gura punch, Akainu's magma or even a Kong Gun.
Comparing One Piece to HxH is like comparing apples to oranges. One Piece is totally high fantasy, while HxH is more of a low fantasy... it has a much closer proximity to our reality.