Did you have the damper on the Guru blower fully open? I always start with mine around 1/4 open to control the initial temp run up from the just lit charcoal. This in my one complaint about a flower pot base in a WSM. Once the temps have spiked like you experienced, there is not much other than the meat to help pull them back down like a full water pan will do. If you can avoid the spike, the flower pot base works great and will make your fuel go a lot longer however.
This is one of the reasons I just stuck with a water pan (ECB charcoal pan replacement) and water on my overnight pork shoulder smokes. Less chance of a run-a-way overheat while I slept.
Here is a photo of the sliding damper I'm talking about. It's the sliding gate at the base of the round portion of the blower. Photos are from the Naked Whiz review of the DigiQ II model which can be found at the following link (it's a pretty comprehensive, multi-page review for someone considering a Guru purchase).
Here is a view looking down the "barrel" of the blower of the 10cfm "pit viper" model with the gate/damper slid to 1/2 open...