I've been running a WSM with a GURU since about 2006 and never had that sort of heat run up issue. You did tape over the other vent holes in the vent where the Guru fan adapter is installed, right (using the high temp foil tape that came with the guru)? Also all the other bottom vents stay 100% shut the entire cook and top vent 100% open the entire cook. You want the Guru to be the only possible source of air to the fire (through that one hole in the one vent - all other bottom vent holes are either closed or taped off).
Also one other thing I do on my WSM/Guru smokes is at the start once the meat is on, I close the slide damper on the guru fan box to only be open about 10-15%. That small slit is more than enough for the fan (I have a pit viper 10cfm) to keep the fire spot on. Much later I will open it all the way (like hours later on an overnight smoke when I notice the Guru fan light is "on" most of the time). Yes, you might see a temp spike when you pull the lid to put the meat on from the inrush of air, but it should settle down on its own in about 15 to 20 minutes. I set mine for 225* and never touch a vent or the Guru control box after I start the cook and let the little computer do all the work. I also start a small pile of lit over unlit with smoke wood chunks in between al-la Minion method for long smokes. That Guru slit damper may be your problem if you are leaving it open all the way the entire cook. Try to close the damper slit on the Guru fan and use a smaller concentrated pile of lit and see if that helps.
Water is good to use until you learn the WSM. Water acts like a natural heat moderator in a smoker especially one running just above the boiling point of water. Ok, physics time.... It takes a huge amount of energy to raise liquid water at 212* to steam at 212*. Way more than it takes to go from 211* liquid to 212* liquid. Also unless you keep feeding energy (heat) to the steam, it naturally wants to fall back to a liquid. So in the process, the water/steam reaction takes a good bit of heat out of the air column around the 212-215* mark. This is perfect for smoking low and slow at 225*. The water slows down any temp spikes, and also helps pull the overshoot back down to that 212* mark just by the way water in a liquid vs a vapor (steam) works. I would suggest you run a full water pan on a smoke where you are wanting to target 225* to 250* until you get the hang of the WSM.
I run a clay flower pot base 95% of the time. It serves as a heat sink like the water does to a point, but it does not have the quality of pulling an overshoot down as quick. But once you get to know the WSM and Guru, you should not really have issues with overshoots so much. Let us know what you try and how it works.
