RLS - That is a lot of space to heat with 1500 watts. The SI units have elements optimized for their size, or maybe a little higher wattage than necessary to accommodate those with an Auber who want to crank up the temperature periodically. But they are definitely not under powered. You have likely noticed that the Model 4 which comes with the Auber standard is rated at 1800 watts. Anytime one opens the door, there will be a heat dump and the colder it is outside the longer to recover time will be. Add in a full load of meat and coming up to temp will be longer than you may be used to. There are very few complaints, if any, about the time it takes a SI smoker to come up to temp or to recover.
I switched from an offset a few years ago and went electric (different pre-SI brand). If I were switching today and going electric, I would buy a SI3. No doubt about it. My brother has it and the quality is second to none, including what I own.
Good luck on your build.
Dave