If the car is jittery at high speeds I would consider what rear toe in you are running. The box settings are for stock, now you have a 19t your top speed is higher. Rear toe-in has the biggest affect on straight line stability.
The oil weight is only going to affect the car during weight transfer (ie cornering and accelerating/decelerating) and bumps/dips in road.
When it comes to bumps/dips in pavement consider also the number of holes in the piston. Too light a oil with a heavy spring = insufficient dampening. I would consider going back up in oil weight and increasing number of piston holes.
i just remembered i still have the droop set for carpet. most likely if i increase the droop by 2-3mm it should help out.
I have some HPI Xtread tyres and they are not very grippy. I would try some sorex28 or softer and see if the grip gets better, tyres are the first thing to look at imho.
i was just doing a little testing/bashing on my neighborhood street, not trying to do much precise setup for racing. i know better tires would help it hook up better, but that will not help the jitteryness.
CF chassis makes the car a lot stiff. you should try softer springs to give the car roll that it needs. And when you feel comfortable with set-up, you'll see that you drive a more predictable car with CF chassis.
if i could only find some softer springs!