To say that favs win 30-35% of the time is incorrect. Favs win from in excess of 80% down to below 20% dependant on the price. Their strike rate goes up when there is less than 8 runners and drops as more runners are added. They have a marginally better performance on GOOD tracks and drop off marginally in distance.
The biggest variable is the price but betting the fav in all price ranges is still going to be negative. Therefore laying the fav should a winner. This is based on the tote and you are going to lay on Betfair which makes it different