Tor,
I always have the same questions myself but I finally got my answer from KOI USA Magazine. I belive it's sept 2003 issue. here is a summary of key points:
1. shallow pond is best. shallow is defined as depth of no more than 3ft.
2. shallow pond has warmer temp. fish grows faster warmer water.
3. fish get sufficient exercise with strong current to retain good body comfirmation
4. any depth of 5.5 ft or more is considered as deep and medium depth is in between 5.5 ft to 3ft. depth of 5.5 ft is considered for pond show case such as tourist attraction
5. take longer to warm up the deep water. hence, fish grows slower in shallow ponds due to warmer water.
6. shallow pond offers better viewing of your fish
based on your follow up entry, sound like you have a good depth pond and good size of water.
Steve