Id like to know if anybody knows the horsepower for a stock 96 Yamaha Warrior? Would a K&N Filter make any noticeable differance? What about a new vented airbox lid?

 

If by "difference", you mean in horsepower, I think not, at least in detectable amounts. However these additions will make a difference in performance by means of utilizing the available horsepower. The use of one or both items will improve airflow to the carb which will, of course, require fatter jetting to compensate.