Sounds and looks to me like you're pretty close to having a decent electric rig. If you're not tossing on a large cut weighing 6-8 lbs (pork butt), it should do a nice job for temps, as long as your ambient doesn't drop much below what you had during testing.
If you preheated the smoker with water in your pan, smoke wood in place, etc, then, toss your prepared meat onto the grate, say a 4lb chicken, you will see a temp drop for two reasons: opening the smoke chamber which vents heat, and the thermal absorption of the meat itself after closing it all up again. The thermal absorption rate will decline in time, and that's when you'll see the chamber temps return closer to where you started. This could take 30-45 minutes, or longer...just guessing here, based on what I've seen with the four smokers I've run, being propane and charcoal fired, 3 verticals and 1 horizontal. I'm thinking that the electric heated pot design will be slower and have a smoother rate of climb during recovery due in part to having lower BTU output, smaller overall size and thermal mass. Heat loss will effect it quickly and recovery may take longer than a larger rig would without making adjustments.
A couple things to remember about using water pans: less water translates to higher temps due to less evaporation of water, more water = lower temps due to more evaporation. Water adds thermal mass, which increases temperature stability, while acting as a temperature limiter. Knowing this, I would say that if you have trouble getting up to your desired smoke chamber temp, then go with a dry pan instead of wet. Dry will reduce chamber humidity somewhat, but part of the cooking process of meats requires internal water evaporation, and if a humid cooking chamber is used, this can dampen the process. Less humidity = faster cooking.
Also, adding clean sand with a layer of foil on top (to catch drippings) in the pan adds thermal mass for more stable temps without the cooling effects of water evaporation. Sand will take a bit longer to heat up than a dry pan, or even a wet pan, but will hold more steady temps than dry, and give a bit better performance for reduction of initial temp loss during opening of the smoke chamber. Total recovery time will be about the same, as the mass of the sand must be reheated after throwing off it's heat from the initial loss, but the average chamber temp during this period may be slightly increased overall, due to not dropping quite as low during brief open smoke chamber conditions.
You may find that using methods which don't require tending during the smoking of your foods may be the best option, unless you make allowances for additional cooking time based on average chamber temps in stead of target or peak temps. I would consider also in holding off for any tending of the meat until I were reasonably assured that the danger-zone temps (41-135* internal) would be within the recommended 4-hr limit. Then, tend the meat to your hearts content.
Maybe I got too detailed there, but I do see a lot of potential with what you have so far. Everything has it's limitaions, and you'll need to experiment a bit to see where the line needs to be drawn, so to speak. Keep in mind that small cuts of meat don't require high smoke chamber temps to cook safely through the danger-zone like a big ol' pork shoulder or beef brisket does. If you can reach 205-210* within 45-60 minutes after loading the food just may well be enough. Quartered chicken may be a good candidate for her maiden voyage as an electric, giving a reasonably large cut, but not so large that it would overwhelm the heating capacity of the 300 watt element. Another candidate would be hot smoked fish...smaller mass, reduced cooking time, reduced minimum finished temperature requirement.
BTW, there are a few 300 watt electric smokers in production for retail sale to consumers (I can't recall brand names or models at the moment), and they seem to be doing just fine with insulated, but much larger capacity smoke chambers than what you have here.
Anyway, nothing seems too far out of line that I don't think you can make it come together...continue foward, I say. Good little project you've got going, and you've put alot into seeing to it that it will work so far. I see good eats coming for all your efforts.
Have fun with it!