At this moment for this tennis season if you took every WTA dog who opened at +200 to +300 (Pinnacle) you are 92-265 (25.8%) and are down $3,996 on a $100 bet each. For the ATP you would be 77-264 (22.6%) and down $7,795 on a $100 dollar bet each.
Now I'd have to go back to chart +400's as I don't keep them daily but dogs from +270 to +300 have performed well worse then dogs from +200 to +269. So I have to think that the +400 and up dogs have to be even worse.
My own stats. Take from it what you will.