You're not "losing heat" as much as you're gaining too much air, stoking your fire triangle (heat-air-fuel) with more air than the fire needs. Every leaky point is basically another chimney exit, pulling more air than is needed through the lower vents, causing the WSM to run hotter than it needs to be. In a perfect WSM the door and lid connections would seal perfectly.
That door is the weakest part of the WSM. The door I received with my WSM was bent WAY out of shape. I took pics, sent them to Weber, and they replaced it free of charge. Honestly, the replacement door wasn't much better than what I had received with the WSM so I don't recommend that path of action.
I've played with both doors to get them to seal better with no luck. There is an aftermarket door available but the payment process was Pay Pal only and I don't use them for reasons I won't elaborate on here. Instead I purchased the sealing gaskets but haven't installed them yet. They have been sitting three feet away from where I am typing for over a month. My schedule will free up in a few weeks then I'll clean my WSM, install the gaskets and let them cure properly.
I'm not concerned so much about smoking at lower temps because I'm more of a hot n fast guy. By closing down all my vents to barely open I can keep the chamber temp under 250F for quite a while if I want a low n slow smoke. The reason I want to seal it up is to save unused charcoal once I'm done smoking. WAY too much charcoal burns off once I shut it down due to the leaks that act both as air access and exit points.