It is not the flex that causes the rub and changing the FD is not going to solve your problem.
Make sure that the high and low limits are well adjusted, angle is correct and there is a proper distance between the FD and the chainring. With that being done, the chain rub will be remedied by using the trim feature of the STI shifters.

Check the manual on the Shimano website on how to use the trim feature of the STI shifters. I have done many adjustments on Sora groups and they have chain rub issues that can almost always be fixed by the trim feature.

http://bike.shimano.com/media/techdo...9830615004.pdf