Oh yes, take the time away from CLK. It's a nice airport, but 8 hours there would drive me mad.

Your bags should be checked through. (Even if you have separate tix, CX can interline the bags through all the way if you show them both tix). This will save you a few minutes.

Caveat: The ferries b/w Central and Mui Wo take about an hour (there are higher speed hovercraft, but they are not pleasant IMO). You will rapidly run out of time if you only have 8 hours. (An hour to land, clear customs, get to Central. Another hour and a half to get a ferry and get to Mui Wo, a couple of hours to get to the Buddha and back, etc.)

It is possible for Lantau licensed vehicles to take you from the airport onto Lantau (after all, it's right beside CLK!). So instead, get a Lantau taxi (light blue) to take you into Tung Chung. You can get a minibus/bus from their to the Buddha without worrying about going into Central, spending the extra time. Alternatively, since you are saving the Airport Express fare, get the taxi to take you all the way.

There is also the A35 bus which runs from the Airport to Mui Wo, where you can change to another bus "up the hill"