I run tyres backwards on the rim often in the race car, in the dry, there is no noticeable loss of performance
However I think you are mad to do the same on a street tyre. There are 2 key things that are different - as Mark said the wet performance suffers because the tyre is not designed to pump water in the direction you are running it. Also it is possible the internal construction may have a problem running backwards eg steel belts loosening.
If it was safe to run these tyres backwards the manufacturer would say so - they only use assymetric or directional design when they have to to meet a certain performance goal.
On a related topic....what is your wheel alignment (camber and toe, front and rear)? If it is out you are better to address the problem than throw new tyres at it.