This is too late, but in case you're going back sometime in the future.
From what I've heard, HK's tailors are not much cheaper than their Western colleagues. Bangkok or one of the major mainland Chinese cities are probably better bets in finding tailors experienced with Western styles at a bargain. The gentleman sitting next to me on my flight back from HK last year was coming back from a tailored shirt shopping spree in Bangkok.
If you want to stay in HK, try the Tourism Board's shop list at
http://www.discoverhongkong.com/eng/...lot_cust.jhtml and stick with the stores with the Quality Tourism Service seals. I'm no expert since I've only gone to a tailor there once. I think I was misled by the store clerk who said they only do one fitting instead of two nowadays. It was a Kwun Kee Tailor in Mongkok.