On carpet there is only one insert that is good enough IMO and that is the Orion/JB Light weight Indoor inserts, Partnr: 70602, JB Foam Insert, JB32 (Blue/Hard)
This is what we use in Europe all the time and it's much faster than "standard" outdoor inserts like the stock ones and Hpi's etc.
They are also available from Peak.....
Mounting them on 26mm wheels makes the car much more stable, but laptimes might be slower so pay attention to that! However the car is easier to drive especially if traction is low...
Try'em out, I am sure you will like them allot!!