Steeping and mashing are two completely different animals. During the steeping of specialty grains there is no starch conversion occurring. All of the starches have already been converted to sugars during the roasting process. All that you are doing is making "tea" and dissolving the sugars, color and flavors into solution. A 30 minute steep is just a rule of thumb to ensure that enough time has elapsed for that to occur.
During mashing, there are heat activated enzymes that go to work to convert the starches to fermentable sugars. Those enzymes require time to get the job done. I can't say with any certainty how much only mashing for a half hour would effect the conversion.