How about 北京樓? There is a promotion offer for HK bank credit card holder right now - a $600 set menu including a 片皮鴨 . This is a 4-person menu but 6 girls can't finish them all.
Umm… if you will go to Kowloon, 鹿鳴春in Tsim Sha Tsui is a very good choice. Advance booking is a must. 北京酒樓 is another good option for its very convenient location near Jordan MTR station.