The WR450F is an enduro bike and it's home is off road in the woods or desert. Any paved road sections should only be used as transfer sections between the off road areas. Any time spend slogging down the open highway will grow old fast. When it's in it's element it will make you happy. Baja Designs kit doesn't weigh much that your really going to notice. You could save money and install it yourself. It's all plug and play. Dealer is making a handsome profit.
The 500 EXC or 690R, IMHO, would be a better choice is your going to spend more time on tarmac. The 6th gear will be much appreciated.