I have ordered from Dharma and Thai silks. Great results from both.
I have used 5mm and 8mm. True the 5mm takes longer to fill with dope but I think there is a trade off with how well the 8mm shrinks back up with the first dope coat vs the 5mm. It seems to me the 5mm shrinks up a bit quicker, for me about 3 to 4 coats of thick dope. I also experimented with the "grain" direction when applying silk. Conventional wisdom says grain wing tip to wing tip. That works fine. When applied the other direction, I get very small "tension wrinkles" along the L/E and T/E planking that takes a few more coats to pull tight. But it eventually does so no real disaster in the making.
Silk is hard to work with but the results are great and certainly worth the extra effort in my mind. Also very strong and never gets those pesky volunteer holes like I have experienced with silk span (when it was available).
Good luck with it. Once mastered, you will be very happy with it.