Hi Antti,
Just to complete this I've made some tests with the "Toshiba USB DVB-T Tuner PX1211E-1TVD".
The short result is: Without PID filter its load is equivalent to the Fujitsu-Siemens. (5.8% and 14.4%) With PID filter it produces even less load than the af9015 with pid filter. About 0.5% with zap and 9.7% with an idle vdr.
Since managing all those numbers starts to get confusing I've put them together on my page at the linuxtv wiki: http://www.linuxtv.org/wiki/index.php/User:Hlangos
I hope I'll be able to add some more receivers.
cheers -henrik
PS : I'll attach the results of the toshiba stick in order to get it archived with the rest of the thread.