I used Soft Seal on a Camaro about 15 years ago and had to add GM's Emulsifying Silicone to the trunk seal to help soften it to where it could close without slamming it. A lot of times it has to do with how the lid is setup, but I tried the repro soft seals again on the green car 2 years ago and had the same issue with them being too stiff for closing.
Switched to GM NOS latex trunk seals from the '80s (non-white string) which was very soft and crinkly and got an excellent trunk seal and a soft closing. Doing that exact same thing with the yellow car I am doing now and recently scored another good trunk seal from a member here for a very reasonable cost.
OER now offers a premium latex door seal and window seal kit that is said to be the best closing product for Camaro doors and windows outside of the costly NOS door rubbers. I have a set to install on this same car but I am a few weeks away from installing this stuff. I will report back when I have the OER door/window kits installed.
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I like solid lifter cars, big cams and cars w/ 3 pedals in them. 
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