We are also headed to WDW in May, the 19th-23rd. I have read the touring plans for that week, and they do suggest there will most likely be a soft opening for Star Tours. The plans suggest that the 19th is the best day of the week for shorter lines and few crowds. We will head over the morning of the 19th, and the morning of the 20th to catch some Star Wars meet and greets, and the special parade.
It is so great when you happen to catch a soft opening. I was lucky enought to be in AK during a soft opening for Kali River Rapids.