I used to do that and in my personal opinion it works reasonably well. With the exception of one type of orders: market-on-close. Liquidity on the 10HKD contract tends to disappear on the close, while on the 50HKD one it's still okay.
If you trade 1-2 lots you should be fine except if you need to execute market-on-close orders. For orders at market you need to size substantially more slippage on the mini. In my opinion, on average - depending on what strategy you have... - 2-3ticks. This is what I had but may be your trading style/strategy will manage to minimize this... The daily volume on the mini is very approximately and on average about 1/4 to 1/3 of the Bid contract.