You nailed it on the head. The MKIV unit has bigger brains

The V28 software(os) and 2007 DVD Maps it just makes things nice and smooth. The MKIII won't go past v22 maybe v23? and won't smooth out the map either or perspective mode or zip code search or map scrolling driving direction. Not like the V28 with MKIV that is.

There are some MK IV plug and play units on ebay in the UK or lithuania for $650.00

There are some locally but they want 1K for it.