ok, if it's a gain of factor 8 in speed, I'll switch to manipulating the ports directly.
btw, my current oscilloscope sucks bigtime. anybody tried one of these yet http://www.rigolna.com/products_ds1000.aspx ?
they're much cheaper than the ones from tek or lecroy, but I haven't decided yet ( 40 / 60 / 100 MHz ? )
edit:
just had a look at the code of shiftOut(...).
I won't be able to do without the for loop, as this a serial connection hence the shift registers.
maybe I can still save some time by removing the mapping between arduino- and real pins.