That would be the quote, ILS. Thanks, Marco. Although I recalled it a bit differently. Interpret it as you will, but Dooku did imply he didn't have much room left for improvement, whereas Ventress, while already powerful, still had many years to deepen her connection to the force and become more powerful.
DP, no, Sidious didn't teach Dooku everything he knew, but I think Dooku was referring to the power he could dish out. Sure he may have been able to learn other techniques, but many techniques require power far beyond some user's potential, or require aid. For instance, Vitiate and a lot of the ancient sith had mastery over techniques, but required aid or rituals, otherwise their own power was insufficient. Mastery and knowledge does not always determine one's personal power. I think Dooku was implying that his own natural raw power was at it's limit, and that he couldn't grow beyond what he had already realized.
Though Dooku is a power house, and most of his feats are done casually, as we've never seen him fully exert himself (unless it's struggling to fight off an angry Anakin's physical attacks or attacks from an angry Savage); however, I'm pretty sure Dooku knows how much he's capable of dishing out if he fully unleashed himself. I'm also pretty sure he had full command of the power he had at his disposal.