The DB-605 that was fitted allowed for a much higher operating altitude than the Merlin 45. However, by the time the test was conducted, the Spitfire XI with the Merlin 66 was in service, and that have roughly the same performance as the Franken-Spit.
Looks like a very clean installation and nice paint job. I wonder how the work environment was in the shop. I bet they had a good time building it.