I am also using “regular” flashlights for night mountain biking. Similar to you, I am using
approximately 1 hour or 2 hour runtime brightness modes. See below for my setup, as an example.
In my experience the helmet light is the more important one. Your S2+ with SST40 will work, but I would want more throw, like a dedomed SST40 or SFT40 in an S2. The required throw depends on your expected speeds, but I don’t live in a mountainous area and would still want more throw for those faster sections even if they are shorter. The S2 is good for a helmet light since it’s light weight. I switched to a 21700 light, but am modifying my setup because it is a bit too heavy.
I think there are less stringent requirements for the handlebar light. It should have a smooth wider beam. Of course brighter is better, but the bar light is illuminating closer objects so it is not as critical for it to be very bright.
An important consideration is getting a light with the right modes. I have my lights setup for roughly 1hr and 2hr modes, but most lights will not come setup with good modes (for me). Often the high mode is too high and the medium mode is too low. I really like convoy’s “ramping” linear driver because it lets you precisely customize the modes. If you decide on a convoy light you might be able to have Simon install it for you.
Regarding thermal limits. The breeze while riding provides a lot of cooling. Also, night riding often happens in the fall when temperatures are cooler.
LOL. What a buzzkill on a flashlight forum. 
Nightriding in the woods is a lot of fun, and can be practical (like when it gets dark very early).