Actually from my limited experience a nuvi works at its best with the original firmware designed for it.
I'm back with US 5.90 no problem to revert either.
Jap firmware works but it's slow and POI search is BAD with loss of Pedestrian mode.
Lane Assist works on JAP but only appears when you are too close to the junction +- 400 m or less which renders it almost useless. Loss of pedestrian mode is also a -.
I've read in OTHER FORUM forum of people "having problem" with de Luxe as well and Alex himself has never advertised that the firmware is perfect.
The best advice is not to cross flash and use the original firmware that was specifically designed for the unit.