We removed the oil injection system on my Son's Blaster. If you do this you must also make sure that you plug the hole in the carb where the oil line use to go. Failing to do this will cause a lean condition and possible engine damage.
The magazines are advertising block off kits for about $30, but you can get all the parts you need at your local Yamaha dealer for about $18.
There is no negative side to removing the oil injection system, outside of the gas/oil mixing. The positive things include knowing you can never have a injection pump failure (hardly never happens), plus you don't have to worry about the pump cavitating (air bubble or foreign matter in the line).
There will be no improvement in horsepower. The reasons most people remove the injection system is for racing purposes. With pre-mix, you know you are getting lube to the engine.