[Savage GT]

I stand corrected and happy to be in this instance :)

Capris and indeed XJS don't require bracing to run well.
On the Capri's I replaced the stock knurled axles with slot.it ones and brass bushings, refit the Scalextric wheels, add some weight on the hatch and some urethane tyres on the rear and some NSR braid on the stock guides.
They drive well 

My XJS problem appears to be a badly fitted stock rear wheel (out of centre on the axles) so I can't upgrade those the same way. 
Obviously starting with an out of centre wheel isn't going to improve by removing it ! I have contacted the scalextric rep on the other forums as the correct axle for the Aussie release XJS is either non existent as a spare, or no-one has one.

Anyhow, as I waited 7 months to get my BRUT car's running mate, a picture is due 

   
Just to wind me up, Scalextric released another livery Capri days after the Spa one   

Interesting to note, there has been a small change in the chassis, the sides are a little higher on the newer version, but doesn't appear to have affected the ability for body float any.