What A2A gives you is a contract for progress and cancellation. What it costs is a larger response (seven times plain HTTP for the same job) and 1.2 to 2.1 ms more per call. The authentication the agent card declares is not enforced by the SDK’s default setup, so read declaration and enforcement separately.
The gateway hop costs under 1 ms per tool call and never improves p99, and in return you get control and observation the server cannot do on its own. One argument-conditioned policy was accepted and still did not enforce what it said, so turn a policy on and then check it with real calls.
Porting the transport to the v2 SDK is mostly a version bump. The real migration is deciding where handle state lives, and leaving it in pod memory brings the old failure back behind an HTTP 200 that transport retries never see.