The only way you will get noticable gains in power is via forced induction as stated above. Any other methods will be wasting your money.
You should change your timing belt. The early 5S-FEs were 100,000km intervals; however, I believe that the later models moved to 150,000km intervals.
As for the leak, valve cover and distributor blanking plugs(early models had distributor) are common place to leak.