The big problem for making pool is shares check....
Example: If you mine 9-er chain and submit 6-er chains as share you'll get about 20 6-er chains in a hour.
Exactly! A miner properly optimized for short chains (to get the best possible reward from pool) would have low performance on 9+ chains. Sort of withholding attack from pool's point of view.
Optimizing a miner to search only short chains is kinda dumb. Let's say ALL the miners adopt it... suddenly no blocks are found and no one makes anything. Face it, why are 5-chains no longer accepted? Flooding the pool with 6 chains will give you a short term increase in XPM followed by either a lockout of 6 chains or a loss of the big dogs finding blocks who get disgusted funding all the small timers who will never find a block stealing all the XPM. The end result of both options is a subsequent decrease in XPM. Greed is always it's own reward.