They are more like consistency changes or QOL buffs rather than actually buffs.
The extra stack on champion kill will only encourage Cho'gath to use his ultimate on champions rather than on minions every 30 seconds.
Realistically, his stack rate doesn't increase, since the CD on feast is reduced by 50% when he ults a minion or monster.
So rather than stacking feast for 3 minutes after hitting level 6 and then spamming the ultimate on the closest champion every time it is off CD, Riot is allowing Cho'gath to consider saving the ultimate for a champion kill instead.
As well, Cho'gath's ultimate already heals him IF he does not have 6 stacks. Giving him the heal even when he has max feast stacks does seem like a buff, but currently Cho'gath would just try to ult the carry at the beginning of a team fight to lay down some burst damage. With this change, Cho'gath might actually save his ultimate for a more opportune moment rather than just using it ASAP as mentioned earlier.