Did Kendo practicing helped Kirito in SAO?
Haven't seen the anime, but at least according to the Light Novels, it is implied that Suguha was better than most other players in Alfheim due to her expertise in Kendo. So answer to your first question would be a yes. It also goes in the other way around, in that skills learnt virtually can be applied to the real world, though there will be limits based on one's physical attributes naturally.

In Kirito's case though he given up on Kendo a long time ago, as kids. His prowess probably came from his inborn reflexes and his belief that both the gaming and the real world are 'real'.