Groundmaster for larger area mowing?

I have about 13 acres to mow, including 6 acres of turf runway for aircraft. I currently use a JD F935 with a 72" deck and it takes me around 8 hours to do the whole lot. I've been looking into larger mowers and wonder if anyone has firsthand experience with the Groundmaster 455 or 580? Things to watch out for, etc. I could use a pull behind type for the runway, but the remainder of the property has a lot of trees to go around, so kind of looking for something that's still maneuverable while covering a lot of ground on the open areas. Thanks for any advice!

Patrick