We have the R package suspension. We put the 500lb springs on the front and it feels good - we may not need a sway bar. We have not tested the car as much as you though. Our car is also not oversteering (other than on throttle), and actually has a slight understeer bias at the limit, off throttle. We also had 245 Hoosiers on though...
With the 300lb springs in the rear, we are hitting our bump stops some, so I imagine that you are too. We will likely need to go to a stiffer spring in the rear (which might mean we'll need stiffer springs in the front too to keep the current handling). If you are hitting your bump stops mid corner, it could definitely break the tires loose and cause a spin. Basically, you'll go from your current spring rate (275# rear), to infinite spring rate, which would cause snap oversteer mid sweeper.
I think, with the 818, that increasing the spring rate until you're not hitting the bump stops would be the first step, followed by either a sway bar for further fine tuning or playing with shocks/springs options more. The good/bad about running e-mod, is that your options for suspension changes are wide open. Then, you get to add aero for further confusion