Torterra has a much better physical attack than it does special attack, so having physically attacking moves (such as wood hammer or seed bomb) would deal more damage to your opponent than the special attacking moves you have listed. Also, if you already have one grass-type move it is generally not necessary to have another, as that move slot could be filled with an attack of another type for better coverage. For this reason, I would replace Leaf Storm for Wood Hammer, and would replace Giga Drain for Earthquake since Earthquake is a very strong move from which Torterra also receives STAB (same type attack bonus).
Crunch is good coverage, as is Rock Slide or Stone Edge - I list rock type moves because all of Torterra's weaknesses and what rock is good against are the same.
Synthesis is viable, as is Curse or Rock Polish, and you can never go wrong with leftovers. Since I notice 252 speed EVs however, I would suggest Rock Polish as Torterra's speed is not good, and can be used to outspeed all your opponent's remaining Pokemon to sweep.
All the moves suggested above can be given using heart scales or TMs in Pearl version in case you have already EV trained it up. You may want to look at a different Torterra set - instead of a fast one you could have a bulky defensive one that sets up Stealth Rock, Curses and uses Leech Seed; it's all about the roll Torterra plays in your team.
You'll notice I tried to answer this more about competitive tips, as good movesets for Torterra are listed here, and very generic movesets are given here at smogon for Diamond and Pearl versions.
Hope I helped!
- That guy who dedicated his username to Torterra