kkck
I can't see the posibility of Coby being a match for Luffy. At the end of the story he can be a significant Marine, but no way a match for Luffy. Even before the time-skip the difference between them was tremendous. Luffy took out Rob Lucci (the real combat genious who had great experience in assasination had tremendous level of Rokushiki that was the greatest in CP9 history and had great control of a DF that suited well to his Rokushiki style), Crocodile and Moria. He also defeated Boa's sisters who were very profisiient in terms of Haki (maybe even more than the majority of Vice Admirals, cause we know that on Amazon Lily people are taught since the childhood), then he went to Impel Down (and don't forget that he trashed Ennies Lobby) and got away (he was only the second in history who did it). Then he got to Marineford and fought quite well (especially taking in account his health, cause he was moving just because of his strong will and with the help of Kuma's hormones).
Also about learning... Luffy was clearly the man who learned faster than it is posible. He learned Soru right from the spot when he fought Bruno on the top of the Tower of Justice.
Also after the time-skip Luffy grew tremmendously. He got way stronger. Before the time-skip all the crew could take down only one Pacifista, but after the time-skip Luffy trashed it with one punch. Even Rayleigh said that he didn't expect Luffy to grow that strong and even Luffy's crew was shocked of his new strength and we barely saw him using his new moves. He took down Kraken with one punch and the Kraken was several times bigger than Sunny.
P.S. I don't understimate Coby or underrate him, but I just see the reality. It was stated in manga that Luffy surely is the leader of new Age and the strongest in the new generation. He surely is a prodigy and has tremendous Haki. His only problem is his dumbness. People overrate Coby too much. If he wants to be at Admiral's level he needs to have DF, cause the only one who was at that level and didn't have it was Garp, but he is surely an exeption. We saw that even old Whitebeard, who was weakened by an illness and his age could take more than one Admiral at a time, so syrely when Luffy will become Pirate King he must surpass WB (I think it's common logic) and should take three Admirals at a time, so no way Coby can be at his Level, cause surely Luffy has a bigger level of growth during the manga.