Rupture is better used where they can't see you

It's funny to hide in bush, hear a cute little stomp, and then feel like you are going up, up, and down.
Now, you want a hybrid to make Cho the most effective in team fights. Rylai's will make your normal attacks (thanks to Vorpal Spikes) capable of Slowing the enemy, your Rupture is great for initiation/disrupting, and Feral Scream REALLY fucks up Mages/Skill-dependent champs. Remember Feral scream silences ENEMIES (notice the S?) in front of him, so this is quite sexy for people who like running in bulks. At rank 5, it lasts 3 seconds (yum); Rylai's would slow them in this case, so using Rupture right after Feral would provide you some dangerous CC. I've done this and it gives plenty of time to my ally Casters to nuke the shit out of my enemies.
On Feast, just get used to the damage it deals. Its cooldown is not that long in comparison to other ults, so this could make it even more viable. There are a few ways to use Feast -
-Psychological Factor: If your enemies get below certain HP level, Feast will secure you a kill, and once enemies get to know this, they will avoid you like the plague. While it sounds like a bad thing, if your carry always hangs around with you... your coward enemies should say good bye to turrets. This combined with Cho's immense size and durability has give me many easy turrets.
-Teamwork Factor: When you know your Caster is around with Nukes ready, use Feast even if the enemy has full HP. Allow your Caster to pop out of the blue and score a quick kill. I can't count how many people got cocky because I was "alone" and suddenly ally Veigar/Annie/Ryze pops in for a swift, easy kill.
-Defense Factor: Building AP Cho will make you slightly squishier than Tank Cho Gath. HOWEVER, since Feast scales with AP, you will still be fearsome (even more from the offensive point of view), thus, making enemies think it twice before attacking you, so even with squishiness, you are still "protected". It is said a good offense (sometimes) is the best defense.
Guardian Angel is perhaps the most annoying item on Cho Gath (for the enemy team, that is). Nothing more frustrating than bringing that +4k giant roach and watching it come back from death, running back to base. You thought you had him...
Anyways, that's just my opinion. He is a great tank and can work insanely well as an off-tank/AP monster. You should play him more often and analyze different options in order to find out what works best for you.