I don't think the truck will flex any further even with the coil spacers installed.
Shock length is the main factor when it comes to flex because it is usually your shock that limits the travel of the axel.
Coil spacers (or longer coils) just make the truck sit higher on the shock's stroke, which in conclusion lifts the truck higher.
If you want your truck to flex more you will need to install longer shocks to utilize your longer coils.
Edit: Doh! TN_Tracker beat me to it
