LE knows SM isn't going anywhere. So why rush the investigation? Make sure all i's are dotted and t's are crossed. Plus, over 200 pieces of evidence? This isn't the only case the FBI is working. I also believe they have some results back which pointed them back to SM. I really don't believe they would go back to a dry well, so to speak, if the test results continued to rule out SM.
I've worked on a case where the FBI was involved and I can say with certainty (unless there has been a huge spike in possibly serial homicide cases) that they have a turn-around rate, at least on the first piece of evidence, of 5-10 days. The GBI turnaround is about the same. MPD has most likely been getting results flowing in since the beginning of the month. Furthermore, the fact that the FBI was involved is in itself interesting. The FBI, under the US code, has the jurisdiction to investigate state matters only in cases where the murder victim is law enforcement, an interstate traveler or the perp is a (or is believed to be) a serial killer. Food for thought y'all.