I have done several changes over shorter intervals to be safer. These heavy duty flushes rather than dissolve gunk sometimes break off larger particles that can get stuck in small holes. Hopefully in time over several changes the lifters should quiet down.
Why 15w 30 oil what brand and mineral, semi or full synthetic? A 10w 30 should be just fine. Been using 10w 30 in my Z and all other cars I have owned and never had lifter or other engine issues.