Use JMeter to issue the HTTP API calls. You will have better control of the concurrency (so you can adjust your load), and the performance report is provided by JMeter. And of course, because you are using Splunk, you can just send JTL files (JMeter result file) to Splunk and let it visualize everything for you.
You can also create your own saved searches (to have dense, rare, sparse). These saved searches will run on cron schedules as specified by you (say, once every minute for rare, once every 5 minutes for dense, etc.), so they will automatically be trigger to run and generate results in Splunk's _internal index. You can then query that index for response times and event counts with something like 'search index=_internal source=*scheduler.log savedsearch_name=myperftest*