IMO, you can drop the hard feed a little bit to compensate for what they're getting from the grass and thats especially at this time of the year. They're not ruminants but they can get quite a bit out of good grass.
We throw our pigs in fresh cut grass from the mower box and as you say, they love it.
The traditional breeds of pigs can tend to run to fat and access to grass can be a way of combatting this.