(conversely less castor will make the car skittish and want to follow lumps and bumps in the road), increasing castor angle will also increase the amount of negative camber on the outside wheel when cornering so you can run more castor and less static neg camber and still get neg camber when you need it - I.e. On the wheel with the weight on it when cornering. Obviously it's a different car, but I run 8 degrees of castor on my laurel, the steering is nicely weighted and it handles brilliantly.
If you have rca's on the front and are using standard track rod ends your bump steer will be negatively effecting the handling massively, the Lower control arm and track rod need to move in the same arc, consider buying some SuperNow track rod ends
(I have them on my car, I think they worked out at £110 delivered from japan) which drop the pivot point of the track rod end down by ~25mm - I have 25mm rca's and my track rod and lower arm are parallel Also, looking at that sheet, your left rear wheel is toeing out and the right rear is toeing in? For a stable planted car you want a smidgen of toe in on both rear wheels which will naturally make the car want to drive it's self forward.
I think I'll probably fit the lower front springs before getting the geo tweaked again though, see how they behave on these shocks 
