I get the way you state it and what is meant, I guess it seems an imprecise way to state it. I still think it would be much easier to state an increase of 2X (old premium is 8,000, new premium is x2 or 8,000*2= 16,000) - after all, it is at least doubling.
Does this also mean that an increase of 400% would then mean an 8,000 policy would actually now be 40,000 (8,000 + (8,000*4))?
That would be more simply stated as “new premiums will be 2-5 times greater than they currently are”.
Like I said, this might just be a funny hang-up of mine as to how things of this nature are stated and the intent of the statement.