Today I did a little testing of my own with Sabledonk against some SP Pokemon. However, I was very generous and had my "opponent" play DialgaChomp and start with Dialga G, Garchomp C, and Absol G. Thats a 100 HP Pokemon, an 80 HP Pokemon, and a 70 HP Pokemon. I put Dialga as an Active, and the other two on the bench. Out of the 8 games I played, I believe I drew into Sableye 6 out of 8 times in my opening hand. This deck is extremely luck and chance based. Although I wasn't able to get a donk, I go very close several times, therefore, if this deck were to be played against, say a Yanmega deck, a donk is almost surefire. Out of the 8 games, I did a lot of testing with having my opponents active Pokemon as Spiritomb. Spiritomb crippled the deck very badly, HOWEVER, I do have a theory. I believe that a donk is still possible, even with an active Spiritomb. You'll have to get a very lucky hand, though. I wouldn't fret too much if my opponent has an active Spiritomb and a benched Pokemon; simply try and Seeker up the benched, get the Sp. Dark on Sableye, and Flashbite for 1 on Spiritomb. Game win.
I do see Sabledonk as a threat to all tournaments, but it's all luck-based. I encountered some times when the game could have been won with the successful coin flip of my SSU, but sadly, it turned out tails. Since I had to take a few cards from DialgaChomp, I could not test it deck vs. deck, but even if you dont get the T1 win, a win is still possible. I'm going to be doing more testing later, but for now, this is all I have.
P.S.: A double Tomb start isnt necessarily an auto-loss either. A Seeker, Flashbite, and Sp. Dark are all it takes. However, it's all luck based.