If the area of the filter is the same then the filter with the finer particle filtration capability will obviously let less air through.

For anyone looking for the same info, I went and contacted dyson directly. The response I got was": "Whilst not all of our filters are HEPA certified, they still filter to the same specifications required to pass HEPA certification. The filters in our machine should last the lifetime of the machine, however they may become worn over time and are not covered under the terms of our guarantee. All of our filters carry H13 classification which means there is a 99.7% particle capture success rate and the filters remove particles down to 0.3 microns. The additional filter on the back of the V6 Absolute, V7 Hepa, V8 and V10 models provides a second level of filtration for the air passed through the machine, this is to capture anything missed by the Pre-Filter or any additional particles blown though the machine after the Pre-Filter stage."

In general, washing is not adviced. See link for tests: https://smartairfilters.com/en/blog/should-i-clean-wash-hepa-filter/

Hard to say when to replace, because it depends on dust in your area and frequency of vacuuming etc. Just replace when you feel suction is much less and/or filter is visually plugged.