You can effectively adjust the flow rate of the 10cfm Pit Viper fan by adjusting the vent on the fan itself. Based on the comments about too much air having a cooling effect, try shutting down the vent to half or 1/4 open. That would decrease the air volume the guru can pump into the drum.
My drum likes to cook at 250-ish temperature range without any help from my guru, which is a pretty good temperature to cook a lot of things. Your turkey would do well at 325 and probably even better at 350.
To get my drum to cook at 300 and above with or without the guru, it takes a good bit more lit coals than I would normally use for cooking at 225-250. I generally use a full chimney of lit coals on a minion method setup when i want to cook that hot. More hot coals will get your temp up where you want it without having to add as much air with the temp control system...