Don't know if this has been mentioned, but the multipliers are now listed on the Champions pages of the Learning Centre.
Now whether they're kept up to date with the patches that come out is a different issue, but they're all there.
Feast - Devours an enemy unit, dealing a high amount of damage that ignores magic resist. If the target is killed, Cho'Gath grows, gaining maximum health (maximum 6 stacks). Cho'Gath loses up to 3 stacks upon death.
Deals 300/550/800 (+0.5) true damage to target champion, or 1000 true damage to target minion (ignores resists).
If the target is killed by Feast, Cho'Gath grows, gaining 90/120/150 maximum health (maximum 6 stacks). Cho'Gath loses up to 3 stacks upon death.
The +0.5 indicates you take your AP (in this case) and multiply it by 0.5 to get the value added.