I was just having a chat with a mate about this recently. On suburban/city roads and freeways, anything over 300kws will probably never be completely used when driving a vehicle within all the road laws.
For me, I think a bolt on only 6L was plenty of power. I now have a cam in that 6L with well over 300kw. Will I use all the power locally? Hell no. Do I need it? Nope.
Its nothing more than a good talking point in my opinion.
But is it fun? Fark yes!