I will only switch the tires when racing. If it is gumbo mud, deep sand, hard packed, wet grass, etc. etc. etc. As for noticing a difference, that will depend on how intune you are with your bike, your skill level and in terms of racing how fast you go, especially in a dh race.

Other then that I will leave my NBX 2.3 FR's on no matter what the terrain when just out toolin' around the trails. Other wise it gets to pricy and is a pain.