Watched your vids from the 22nd of November.
A few things.
I would raise your ride height as I can hear it chattering over the bumps coming into some of the turns, even over that wind lol. I would also look at having more droop say about 2mm to 2.5mm over ride height. (Set your ride height to 6mm, then screw the droop screws out/in so when you lift the rear of the car by the shock towers you reach a ride height of 8-8.5 mm, just beofre the tyres come off the ground)
Your brakes seem to be too strong also and are locking up as your brake, this is where the car gets a bit of its over steer. I would also gear up, on the back straight it sounds like it is revving out pretty hard which means you have low gearing in it. Low gearing will also promote more wheel spin from out of the corners as it generates more torque. Maybe also adjust that throttle finger also
For 6 degree temps, I would be using a very low range temp tyre like a sorex 24. I would suggest tyre warmers too and bring them up to heat at around 60 deg for 10 mins.
Adjusting the fron to rear roll centres will not do any harm at all. Essentially a car has four basic roll centres. One at the front, one at the rear one horizonatally (front to back) and one vertically (up and down, though thsi is not really a rollc entre it own right but the effective sum of all of the roll centres acting on each other, dynamics, not static.). In terms of roll the best way to think about it is "every action has an equal an opposite reaction" Ie if i place more load on the left then there is less load on the right (play with your car and push down on the top left of the shock tower), well in terms if things are equally balanced, ie symetyrical.
So when i suggest lowering the rear roll centre you are basicall angling the chassis downward towards the rear of the car. This reduses the weight transfer from the rear of the car to the front of the car across the vertical roll centre, also known as "jacking" (commonly a karting term) ie the end of the car lifts up.
This is where dynamic forces come into play, so imagine if you are braking the rear of the car jacks due to a too high roll centre and then you turn. The lightest end will want to rotate around the front directional axis and the rear will lose the grip.
Now with the front roll centre essentially it is the same as the rear, however it is generally the main pivot point for a vehicle. Basically the front roll centre can have the largest effect on the amount for front grip A high roll centre is best for high grip tracks due to the redued amount of cambergain and effective larger load that a high roll centre promotes, the car feels stiffer. Where the lower roll centre promotes a larger amount of camber gain that gives a softer feel and allows more grip on low traction surfaces. As a rule of thumb for me, if it understeers too much go lower, if it turns to sharply or traction rolls on the front go higher. This can also have a lot to do with springs in the front also.
So the best setup is the right spring rate with the right dampening oil, coupled with the right roll centres front/rear and front to rear for the amount of grip that the track and tyres can give.
Its a cool little track that you have there and intersting tunnel, I bet that can cause some carnage.
A bit of a lecture I know, but I Hope I have passed on my knowledge to some people and it helps you in some way.