I am trying to run a test with 11 steps, and I want to define a curve of how frequently tests will be sent. That is, I want to start off with one request per second, then graduate that until I hit a maximum of 15 requests per second, sustain that for a period of time (say, 10 minutes), and then end the test. Is this possible or is the only way to start at that number of requests immediately?
All possible load strategies are listed under Simulating different types of Load SoapUI documentation page and no one really fits your needs.
So maybe it worth considering converting your SoapUI test to JMeter, this way you will be able to use Throughput Shaping Timer and enjoy more comprehensive test results in form of the HTML Reporting Dashboard