Even the non-turbo 300D is pretty sluggish (not that the turbocharged version is anybody's speed demon), so your hopes for the 240 to save you from the molasses-like pace of the 200 are pretty unfounded.
Yes. But in this case, the 240D engine would be running with a 200D's 3.92 rear axle ratio.
Since we on the Forum can't actually test drive this car, perhaps we need to try and determine what 'slow' is. What kind/grade of hills are you having problems with?
Typically, what speed and in what gear are you running on these hills?
I have a fairly steep grade on a local road, with a 45 mph speed limit, that I have no problem maintaining about 45 mph with my '82 manual 240D. Now if I had to stop before this hill and regain my speed, I would be able to upshift to 3rd and make it to 45 mph eventually. Keep in mind that I'm running the stock 3.69 axle ratio. A 3.92 would be a significant improvement in power, but would somewhat affect my high-speed cruising ability & economy.
On interstate trips, and the usual grades there, I've had no problem maintaining the 65-70 mph speed limits. Though I generally cruise well below those speeds on local freeways where the limit is 55. There's a local highway that goes over a mountain, that I haven't yet tried in my 240D. But I expect I might have to downshift there. I don't expect to zoom up to the top at 65 - 70 mph like gas V8 vehicles.
Again, my 240D has the stock 3.69 rear axle ratio. Not the 3.92.
With a healthy 240D engine, you should notice a significant improvement with your engine swap. Whether it will be enough will ultimately be your experiment. Keep us advised.
Happy Motoring, Mark