My KX used to rail berms like nobodys business. .
You need to spend a day with the new KX450F (I did). You will throw your 500 in the trash.
KX500's still have their solid purpose in off road, but as for the KX500, IMO, its NOT in the berms. Bottom line you just simply can't easily transition foreward on the seat.
as for your steering stabilizer, you are probably right, it might come down to purchasing one. I have gotton TONS of nothing but positive feedback on stabilizers. Heck, I even know a few MX guys that run them!