According to my manual your device already has LA,JCV and Speed Limit as well as Routes.
I have managed to get MP3 player and Routes working by loading JAPAN firmware 360 for 1480 combined with US 580 boot.bin (on a nuvi 1300)
You lose some functions like Language Guide and Ability to display (More Data) on Nav Screen.(There are also a few unusable icons.)
You also have to use RGNVoiceEditor to add support for TTS Voices.
The POI Search is also not as nice as US.
Changing to JPN will not really be worth it for you, I think you lose more than you gain.
There is also the risks involved.
You could look at Purchasing de Luxe Firmware, I don't know the specs ? but you can Google for it.