I'm honestly curious how CP0 is going to be a relevant threat to the Straw Hats.
Enies Lobby was a great arc, and Lucci was the first physical hurdle Luffy had to overcome, IMO. Until then most of his opponents were difficult to beat due to Logia abilities / trickery.
CP9/CP0 = CIA / KGB type org.
Admirals/Marines = Army / Red Army type org.
What that tells me is where Akainu / Kizaru / Fujitora are brutish warriors who engage in straightforward tactics and have insane offensive powers, the CP0 agents on average will have a diverse array of abilities that assist in espionage and assassinations. I expect abilities that are in general, hard to deal with. Luffy is not great at "tricky opponents" and unless he can straight up bash his way through those fights are better suited for his problem solvers (Robin, Nami, Usopp).
If you think about it Akainu isn't out making deals with Yonko. He's commanding Marineford and the whole force of the Marines in the New World (and Paradise, but that's less relevant). CP0 is out making black market deals with Yonko, and engaging in horrible trade such as buying children (Linlin, for instance). They're sending assassins to Impel Down to assassinate Doflamingo for what he knows. They're sending spies to Big Mom's Tea Parties, and agents to oversee Kaido's war.
It wouldn't be unreasonable to expect that Lucci had a growth spurt similar to Luffy in the two year time skip. Sure, Luffy trained with Rayleigh, but Lucci also had resources available to him. I can't imagine CP0 doesn't have some harsh new-agent training and mentorship. The marines in general have the likes of Garp, Akainu, Sengoku, etc, to help the new age grow stronger. For all we know Lucci had some super CP0 guy train him for two years. CP0 was able to push Blackbeard out of Baltigo and we don't know the circumstances of that, just that it happened. The Tenryuubito are people who can summon an admiral in the instant they feel threatened. The Five Elders have the confidence to ask Im who Im wants "erased" and they seem fully confident in their ability to carry out Im's wishes. I highly doubt they are about to dispatch Akainu to do so. They are going to use CP0 agents. CP0 has more knowledge about what's actually going on than the Fleet Admiral himself. Certain obvious orders from the Tenryuubito that outrank Admirals themselves have been witnessed in the manga (the razing of Ohara, for instance).
CP0 is an organization that has ties to the likes of Kaido and Joker. They meet with brokers like Mother Caramel to purchase promising "future admiral candidates". They infiltrate Yonko events to gather information. They infiltrate Impel Down to assassinate high-risk prisoners. They attack Yonko when necessary (Blackbeard) and are considered a threat to even the Revolutionary Army. They literally hold a Kingdom back from saving their own Princess (Shirahoshi) from slavery at the hands of a single immature Tenryuubito.
With how this all relates to power levels - many of Luffy's enemies trail off in the distance for whatever reason as he grows stronger and stronger. This isn't the case for everyone. Crocodile showed he was still able to get around Marineford and hold his own. Hell, he even attacked Akainu. Lucci had to escape the WG with the rest of CP9. He's a literal legend in the WG as a hardened, badass spy who can take on anyone. He hasn't been sitting in a prison, or being all "Woe is me". He's been doing active work for the WG and growing stronger. CP0 missions don't exactly sound easy (see previous paragraph). He has likely grown organically, similar to Luffy. I think Lucci could potentially be a serious threat, still, especially to a weakened SHP crew.