“What if a group of hard as nails criminals had to become women to rob the bookies at Royal Ascot’s Ladies Day event?”

How's this?

After losing everything, a desperate ex-con poses as a woman during Royal Ascot's ?Ladies Day? in an attempt to rob a crooked bookie and win back his family."

I agree... starting a logline with what if, seems to general, like who are you asking?

I wouldn't write a logline starting with What if? because it really only pictures a situation, not a story.
In this case I believe you can improve the logline by answering the following questions:
1. Who is the central character in the group of criminals? Don't do multi-protag (http://bit.ly/ONEhero)
2. How is it different from / better than SOME LIKE IT HOT??
3. Why do they have to become women? What happened? Or in other words: what is the inciting incident or catalyst of the story?
I hope this will help you in refining the logline.