Before the extra insulation was added there should have been an air sealing effort. If they took the insulation level "to r-30" as opposed to adding r-30 to what was there then you have a minimum level of insulation, but there was probably a difference. To see the difference it would be in the cost of cooling and it gets a bit complicated adjusting one year to another based upon the weather. If the added insulation was a neat job adding more would make little difference. Only time I find it cost effective is to add more while they are there, very little added cost.
If you post a picture of that attic it might help.
Bud
If you post a picture of that attic it might help.
Bud