The 15% cap likely only applies to IRS-reportable gains on the congressperson's personal tax return. That unfortunately doesn't preclude insider trading by spouses, within IRA accounts, or within wholly or partially owned c-corporations controlled by the congressperson or a close family member.

We need a federal law that says: "the definition of material non-public information (MNPI) is extended to mean any non-public information those in federal, state, or local government are privy to that may affect securities prices, and individuals in or adjacent to government are equally subject to prosecution for trading on it".