Any deviation from stock potentially decreases the value of any car and almost always the value of "collectibles"(which an ImpalaSS is). You narrow your potential market. But at the same time it depends on how you look at it. Certain mods, well done, will gain you value in a place like this where you're surrounded by fans of these cars. Especially now when the prices are down in the general market because they are seen, by most people, as just old cars. The 'net, in letting people come together over a single common item like this, has altered things...at least some.
OTOH if your consideration is long term collectibility then other than wheels and easy to change bolt on items it's always best to leave it alone. A matching numbers, original color 1st Gen Camaro, for example, either nice original or well restored, is worth more than any custom with few exceptions. Even when a custom does bring more money it's harder to sell it because, again, the market is narrower(and it's often pretty narrow to start with).