The 2mm will make a difference.

Don't think of it in terms of pure numbers, think of it in terms of percentage .. or more accurately look at the FOV captured by the two different focal lengths in terms of °

The percentage difference between 8mm and 10mm is about 20%, whereas 2mm difference at say 100mm(compared to 102mm) is only 2%.

FOV is where you will see the bigger picture so to speak.

The 8mm(on Pentax APS-C) gives a 114° FOV, whereas the 10mm only captures 102° FOV.

Question is:
* would you notice it?
* do you need it?
* is it important?

Issues to understand too.

Can you easily mount a filter if need be?

While many people see CPL's on such UWA lenses are more problematic, there are still benefits to be had in using CPL filters even on UWA lenses .. it's not always only about getting a blue sky!

You could eventually modify something to mount filters onto the 8-16mm, but the 10-20 does it much more easily.

The other aspect to be weary of with really really wide lenses .. it's harder to compose your images with them. Not impossible .. just a bit harder at the super wide end .. and in many situations, you will probably zoom in a touch to compensate if other options aren't easily achievable.