How is the downforce 'suffering' and how do you know it's the underbody?
I'm looking at the line of stagnant air betwixt the rear wheels. It looks like the transition to the diffuser is too sharp. If possible, make it longer and flatter with some serious fencing. The outer fence can be right up against the inside of the tire.
In the front, the front wheelwells are always the worst. Short of front wheel skirts, airdams and spats are helpful.
