that mechanic is a moron. a little too much air does not make the engine bog. too much air makes the motor run lean which would do the opposite of bog, it would take away your top end power in all of the gears. The only way the engine would bog from too much air is if it is not getting any fuel, like if the jets were clogged or something.
You can leave the K&N on there, although it is not a recommended filter since it does allow lots of tiny dust particles into the motor.
bogging engine would be caused by running too rich.
did you take off the airbox lid? believe it or not, taking that lid off actually allows less air into the motor which will cause it to bog.
If the air lid is still on there, then it must be jetted a bit too rich. There are some things you can do to lean up the mixture so you won't get the bog, or you can have it rejetted again with a leaner mixture, or adjust the needle.