KMS access is granted by either environment variables or by authorizing the instance itself. Either way, if the instance is compromised, then so is access to KMS. So unless your threat model involves preventing the government from looking at your data through some theoretical sophisticated physical attack, then your primary concerns are likely the same as running a box in another physically secure location. So the same rules of needing to design your encryption scheme to minimize blowout from a complete hostile takeover still apply.