Hi Rolf,
I had the same question as well and didn't really get an answer anywhere about practical differences.
I bought mine from Orion optics uk, and they have 4 grades from 1/4pv wavefront to 1/10pv. as for how much of a practical difference it makes with deep sky imaging, I'm very curious to know as well.
I asked them this question and they were more than confident that even their professional grade would yield far better results than standard GSO mirrors for DSO's. so maybe it does make a difference.
there is a good write up here
http://www.orionoptics.co.uk/OPTICS/...topticspa.html
I will be testing my 10" soon with the same camera that I used with the GSO mirror. will be interesting to see the difference. I'd be very disappointed if there isn't much.
But generally it makes a larger difference with planetary imaging.
would love to hear what others would have to say.