As far as I understand from various posts the HD filter is a necessity if one is cutting dry hardwood because of the dust like chips generated by doing so. The bigger surface of the HD filter allows extended cleaning intervals.
It also seems recommendable to get the HD filter if one has ported his PS7900 to ensure better , less obstructed and more consistent air flow.
I am sure I will be corrected if I am wrong with my assumptions!
I have a Dolmar PS6400 (well used) and a Dolmar PS7900 (has yet to prove herself in big wood) , they both have the stock air filter and work flawlessly.
I played with the thought to buy the HD setup but it is just too expensive running at around 60Euro in my area!
For my purposes the HD filter kit is pointless!
I almost exclusively cut fresh wood , meaning
really big chips , and I thus far never found my stock (furry/filz) filter being clogged upon inspecting it.
That said , it's just my opinion , it mostly depends on ones needs - each to its own!
NOTE: I like the look of the black HD filter setup , but it is not worth my hard earned money just for that!
I noticed that I like
black stock filter hoods just as well , thus I plan to buy a black "normal" hood for about 7Euro just to please my eyes.
Enjoy Your Dolmar PS7900 , I sure hope I will!