I don't know much about offsets. I've built a few insulated drums but never built, cooked on or used a traditional offset. I do have an offset Brinkmann in the back yard that's "on the list" but it was given to me and I've never lit a fire in it.
What are you using for fuel? (charcoal, sticks, combination, etc.)
Have you ever heard the saying about cars: "there's no replacement for displacement" Something I've learned from building and cooking is that mass is a good thing. More bigger/heavier is more better. Not bigger in terms of cubic volume, bigger in terms of the gauge or thickness and the amount of metal used. Lots of people will put fire bricks in the bottom of their offsets or sometimes people put bowls full of sand in drums between the fire basket and the meat. It's like a flywheel effect, the mass of the fire bricks and thick metal (combined with good insulation and not opening the cooker) takes longer to heat up initially but then it's got some inertia behind it so when you open your cooker it maintains temps (or rebounds to ideal temps) better/quicker.
Heat rises so with a traditional offset I'd think that controlling the chimney flap/vent would be crucial. If you keep that thing wide open then yes, that side is always going to be very cold. Remember, this is all about indirect heat, if you already have a tuning plate, nothing is stopping you from plugging up a few of the holes by firebox with aluminum foil and experimenting a bit...
My only other comment would be about "knowns", "unknowns" and "known-unknowns". (variables) Maybe everyone doesn't agree with this but to me, it's OK to have a 150-deg temp difference in your CC. As long as you KNOW the temp differences (and they are repeatable) then you are good. Honestly it gives you more flexibility: hot & fast on one side and low & slow on the other. Or you can balance different protiens so they all finish roughly around same time, or when you look at a cut of meat you can put the thin part on the cool side and thick part on the hot side... You can balance it because you know what is going on inside the CC.
Unknowns are bad. That's when we chase our tails, open the CC constantly, adjust everything quickly thinking it'll make a difference, etc. When that happens to me, everything I do/try seems to make it worse and usually takeout/pizza ends up happening those nights.
Known-unknowns are things like: I know I have a 150-degree temp differential in my CC with brand X fuel but I've never used brand Y fuel so I KNOW there SHOULD be a temp differential in the CC --but I'm not exactly sure how much it will be with brand Y fuel. Make sense??
Our bbq team had a pretty crappy offset for a little bit that we used for catering. We just shoved tons of thermometers in there and then cooked as much as we could (personally, for us/friends/family, before we catered with it). That way we could establish the rough temp mapping of the cook grates. From there things got easier. Once we had a rough layout of the temp differences we started piling in fire bricks in different places to even out (or direct) the heat.
Hope that helps.
Oh also some fuels burn hotter, cooler, longer, faster, etc. so in addition to all of that, experimenting with different fuels may help too. You may find that a cooler burning fuel keeps your chimney end of CC same temp but lowers temp of firebox side. Also maybe try shoving a fire brick inside the FB up against the CC, often times with offsets the heat from the actual fire can transfer through the shared metal surface between the FB and CC. (or at least pull your coals away from the CC end of the FB.
Good luck and happy cooking.
