I doubt that 3-3-5 will help you if your looking for more steering. Generally a heavier weight oil in the rear will cause you to push more.
I have tried 5-5-3, 4-5-3, and 3-5-3 recently and 3-5-3 has been the nicest set up for me as far as diff oil goes. I'm racing on a clay indoor track. I'm running 17 degrees of caster (1 dot down) and find that to work very well. Also running the inner most hole (less inclined) on the ackerman seems to improve response time.
I agree on the swaybar setup too - i'm actually running the softest 2.2mm swaybar and it feels pretty good. You could also try just ditching the front swaybar all together.
I dont know how you didn't noticed a change in 4 degrees of caster. You may need to look elsewhere for binding problems or something. My biggest suggestion is to just go out and start experimenting. The stock setup for the XB8 felt really good for me but I knew it was capable of more. I took about 2 weeks trying alot of different things to just learn the buggy and see how it reacts to these changes.
This buggy will do anything you want it too
