I have been to Thorpe park 4 times since opening and stealth has run perfectly when opened, if its gone down during the day its been only for 10-15 minutes. I was also there on Sunday and there was a rollback and it was tested for ages, then one train started loading and the other train was still being tested, 20minutes after the rollback. This i cant see why as other rocket coasters just relaunch until it makes it over the hill. Also on 3 occasions it has opened late around 30mins after park opening.
I think Stealth's downtime could be a lot worse and i think it has run very well since opening.