What is best way to performance and load test AWS cloud applications

Want to keep this question generic and expected answer in terms of best practice/approach/guidelines,

We need to know the best way to performance test and load test AWS cloud based applications.

What we have tried:

We used Gatling and Jmeter to execute our performance tests. These frameworks are pretty useful to test our functionality and to benchmark our applications latency and request rate.

Problem:

Performance benchmarks and limits of AWS managed services like Lambda and DDB are already specified by AWS e.g. Lambda concurrency behavior and DDB autoscaling under load etc. AWS also provides high availability and guaranteed performance of managed services.

1. Is it really worth executing expensive performance test and load test jobs for AWS managed services?
2. How to ensure that we are testing our application and not actually testing AWS limits which are already known.
3. What is the best practice and approach to performance test cloud based applications.

Any suggestions will help tremendously. Thanks,