Just went and had a quick look at the celestion site,
to find out the exact difference, and here's what they say:
What’s the difference between 100W at 16 Ohm of impedance and 100W with an impedance of 8 Ohm?
100W is still 100W whether the load is 8 or 16 Ohm. The voltage and current will change according to the load impedance and the amplifier design, but 100W into 8 Ohm is the same as 100W into 16 Ohm. You will probably find that your amp will deliver more power into 8 Ohm than into 16 Ohm.
All this stuff and a bit more is under their guitar loudspeakers/technical faq page.
It doesn't really answer the question directly though, but as it says, 8ohm's worth of speakers will have more power running through them overall (and correct me if I am wrong) this should lead to quicker speaker distortion. (but I may be totally wrong there, and if so will edit reply once someone comes and puts me straight!)