Update: Today, I pulled the trigger on a very lightly used (home only) Tyros 4 and speaker system for £1,500. Picking her up Monday.

In the end, I decided I just couldn't justify spending twice the money on something which is only going to play a secondary role in my work pattern (at least for now). Having watched lots of Tyros 4 videos, I concluded that there was plenty of inspiration in the T4 to keep me busy for some time. It also means I can afford to keep the Kurzweil, which is a very capable and convenient board to have.

Once again, thanks for all your input. I'll tell you if I made the right call in a few days!