You don't want to use a filter that is much longer than stock. Even the stock one is sometimes vulnerable to flying debris. A factory race bike was disabled by a punctured filter when they were racing the Daytona a few years ago. And, I suspect that some automotive filters are made from thinner stock than bike specific ones.
Another thing to watch out for. The glue used in assembling the really cheap automotive filters might not hold up to the higher temperatures encountered in air cooled engines.
