One would think if the blow holes did not add much a pencil pusher at the factory would realize they could have lots of man hours ($) by just using dimple tape.

On Monday, October 24, 2016 at 11:30:06 AM UTC-7, soarin wrote:
At 16:50 24 October 2016, Jim White wrote:
Herr Waibel has been quoted as saying of the ASW27 that "there is a 50/5
chance that the blow holes improve performance by 1/2 point". Possibl
apocryphal. Easy to clean with a piece of copper wire and a vacuum cleane
but tedious beyond belief. jim

I once got a DG300 in for insp and found that the blow holes on one wing
were inop (apparently the seal of the internal blow hole channel had
failed). The factory said a fix was problematic and to go ahead and
install dimple tape as it works just as well. No telling how many years
the glider flew with one wing turbulated and the other not. The owner was
oblivious to any degradation in handling or performance before and after
installing the dimple tape.

ME