Came across this forum post that points out that suspension and roll bar settings depends on the weight of the car at the time they're added: http://forums.forzamotorsport.net/turn10_postsm431464_Forza-...

I haven't been able to try it out myself, but apparently suspension and roll bar settings are calculated from the weight (and weight distribution) of the car at the time the upgrades are added. This means you're more likely to get a decent starting point if you add these at the end, after you've done all the weight reduction (you'll probably have to leave a few spare PI available). If you've already added them, you can apparently just revert back to the stock suspension and roll bars and then reinstall the race suspension and roll bars to get the updated settings (similar to how tyre pressure is reset if you change tyre compounds, which has caught me at least once).