As you read the entirety of this post, please keep in mind I am respecting your love for painful honesty. If you were a more sensitive person, I may not be so blunt. But I know you invite the untarnished truth, so here it is:
If you don't know what will set this book apart for potential readers, why write it? The first thing a writer must know is the value of his or her book for the reader. What do you want the reader to feel, experience, and come away from your book knowing?
Asking an audience what they want in a book is like a college professor asking his class what they want to learn. It is a trademark of The Author that he or she KNOWS what the audience wants, usually through experience.
Today's nonfiction audience wants information and instruction presented to them in an entertaining way. The best nonfiction authors borrow from the art of fiction through characterization, suspense, and tension.
I would submit that your potential audience wants authoritative and proven instruction on how to get grant proposals funded through preparing better proposal documents. And the probably want something new, something they have never tried or heard before. I know I do...
YES, the words "like a professional" would turn me off. I am already a professional. Reading the words "like a professional" suggest to me that the book is for amateurs (i.e. those who are not professionals). If your book is for executive directors and fundraisers (people with experience), I would suggest you capitalize on what is new and different about the book. However, if it is for amateurs, you would be perfectly safe including "like a professional" in the subtitle.
If you have not done so already, take a couple hours and drive to 2 or 3 Barnes & Noble or Borders bookstores in your area. Look at the grant writing books on their shelves. Notice what is similar about them all. Then make the title and subtitle of your book the exact OPPOSITE.