Shield Penetration might not be the best example for this - I mean it is basically capped at 100% on a logical basis.
100% Shield pen = You ignore Shields.
As for the other stats, say Attack Speed, each character should have a theoretical cap based on helix and highest possible gear stats.
But I don’t know if there is a hard cap in the game for this - and it might vary on other circumstances.
For example, there actually is a cap on +Max Shield for all Eldrid chars, that basically only affects Kelvin under normal Circumstances.
(Source: Tips and tricks (Mechanics you may have missed))