soft on the bumps? 8 scale cars always feel a bit soft compared to other cars. If it isnt a problem I wouldnt change it that much.
I would try a shorter position for the upper rear link (might be called upper rear arm) to generate a bit more rear end grip.
If you dont already have done so I would use the inner hole on the wheel side if thats not enough you can use the lower inner hole on the shock tower too (if you use the higher position at the moment.)
If thats still not enough check the diffs and the shocks first
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What diff fluids do you use? I would start with something like 7000 front, 5000 middle, and 1000 rear (or just some grease for the rear diff). If you want to calm the rear a bit down you can use 2000 for the rear. If you run on high bite tracks you can use more viscous oil all around with same proportion.
edit: shocks are super important but if you use common oil (400 front 300 rear for example if I remember right the mp7.5 shocks with stock pistons) I wouldnt start with changing springs if there is a lack of rear end grip. Those stuff is hard to tell on the internet so others might recommand to do so first but on 8 scale I used to soften both rear and front at once I did not really use it to set grip level between rear and front but this is just how I did it. For those stuff I would ask at the track.