It can sometimes be a good idea to toggle off Vorpal Spikes to prevent yourself from pushing the lane too far and thereby opening yourself up to ganks. If you don't toggle it off when attacking a turret, an enemy champion can stand near the tower where you will hit them due to Vorpal Spikes'area of effect damage, causing the turret to attack you.
If you're having difficulty Feasting upon champions, try eating minions until you're larger, as the true damage you deal exceeds the health of most minions. It is advisable to gain 6 stacks first by Feasting on minions and using it to last hit enemy champions that are too close, since the damage to minions is significantly larger (particularly in early-mid game, since Feast have a base damage of 1000 against minions, regardless of Feast's level).
If you want to push your lane faster, it's better to Feast a Siege Minion for an instant minion kill since they're harder for your smaller minions to kill.
If your team has a tank already, Cho'Gath can switch to an offtank ability power build in order to do more damage. Since Feast already increases Cho'Gath's maximum health, he will still be difficult to take down.