mine (side panel fan 120mm ducted straight to Alpha PAL8045 on an energetic Palomino 1600+)
No filter: 48°C
large pore filter: 49°C
small pore filter: 50°C
I had the same question as you a little while ago, so I ran the above test real quick. All temps are properly thermalized Sandra CPU mulitmedia burn-in for at least 1/2 hour (takes mine a while to thermalize).
The small pore filter clogs up real quick, but saves having to dust the HS ever. Hope this all helps.
N8
PS heard pantyhose, dryersheets, all sorts of stuff work just fine and wont effect temps too bad, as long as you clean them as needed.