As long as the donut and mating surfaces are in good shape, you shouldn't have a leak. Are you sure (I have to ask) that you installed the springs in the right place...i.e. bolts go thru the springs first then the bolt/spring combo goes thru the holes from the q300 side and screws to the midpipe, so the springs are pushing the Q300 pipe against the donut?
If they are installed correctly, remove them and verify the 2 sides fit together relatively well, there shouldn't be a whole lot of pulling going on.