From what I understand if you get the extension tube you can bring up the wattage to 120 if you are in wattage or non-TC mode. However, if you are using TC mode you will still be limited to their 50 joule limit, extension tube or not. The advantage is just more battery life and not more joules in TC mode.
I have wondered why Yihi made a 50J limit in TC mode but then after using the device I rarely go above about 34J anyway. If you are setting your temperature setting somewhere in the low to mid 400 range as I see most folks have been doing, it would serve little purpose to have a higher joule setting. The joule setting appears to only be used to get you to your temp quicker if you set it higher. Once it reaches TC the chip takes over to limit the power. That being said, with the powerful and powerful + modes you could receive that same effect without having to have a high joules setting.
Another point, being sick with a bad cough I am having difficulty vaping without getting into a coughing spree. I have found that using the soft mode eases in the vapor more gradually rather than the other modes. Using this mode I find that I have less tendency to have it trigger a coughing sensation. When I first got the device a few days back I preferred the Powerful mode. But that was when I didn't have a cough. Just something to think about and their various modes.