Good to hear. You want approximately 1 to 1.5 seconds of slip from a full throttle standing start. It's what we tend to run on astro anyway, anything less and the car can become very snappy and unsettled when exiting corners. You have an older model but still a race buggy so I can imagine it'll out drive most RTR cars.
For a grass setup, it's very hit and miss due to the lovely British weather and how long the grass is. I don't race 4WD or on grass but I'd suggest if you're understeering a bit, raise the ride height, 21-23mm and soften the suspension (probably thinner oil, 400WT front and 300WT rear). This will generate a bit more corner grip by allowing the car to rotate more through a bend. The rest is up to you and how you want the car to drive.
Enjoy