I would say that today I was close to your weight on the bike. I am at 220 and I had a bike rack on and some gear stuffed in it. Had to be darn close to 250. It was my first ride on the bike. The semi-trail tires did better than I thought they would. I had them at 60 PSI. I cranked up the rear shock for my weight. It is pre-load adjustable.
Can't do anything with the front but it was soft. I may be able to disassemble it and pre-load it more but no time yet. Rear felt pretty good. Bike tracked well in the snow. Rear derailleur is NOT adjusted properly but it was too dark outside to see what to set on it. For example when gear shift indicates between 1 and 2 it is already off the 1st gear cluster and going into the little plastic safety guard. It was hunting between gears but all in all it should be easy to dial in when there is light out.
I stopped to try and do it but my hands were just too cold. It was about 15 degrees outside. I will need to tweak other adjustments such as seat. Speaking about the seat I was surprised. It seemed pretty decent. I guess the way to describe it is quality wise it feels about like 80% of a Dahon but at about 40-50% of the price. This makes it a screaming deal in my book. I have to unbox my other one and see if it is as good.
I do have a few nits but all in all a pretty good piece of kit. It is a better bike than the Boardwalk for about the same money. The suspension while old tech is appropriate for this bike. The weight is heavier than my Dahons but they are the top models in their ranges. Nothing that I would really say is "cheap" on the bike. I don't like the foam grips that much but that is easily changed. Rear derailleur is a little stiff but I'll see if it breaks in. Brakes seem ok to good. One rim might have a seam or something that makes a little thump each revolution so I'll check that out with more light. My wheels were true and spoke tension good.
I don't want to seem like I am slamming it but just giving an honest unbiased assessment. I'll be adding fenders, kickstand and a Pletscher rack and more permanente lights. I will add better tires but so far it's a pretty good bike. I'll buy a couple more for my family probably.
I can't speak for the design limits but most bikes have a safety margin. Don't jump it or do foolish things and most frames can handle it. Just keep an eye on spoke tension. I was at the upper limit for my Dahons and never went with the ultralight models just for the weight reason. Even then my spokes were a constant problem on my SpeedPro until the finally started to bed in. I still checked them every 2 or 3 rides. Now the Downtube IS NOT a lightweight bike in anyway that I can see. It is close to 30 pounds but a nice cruiser with decent gear spreed. Better tires and it would be moderately fast for me. For 90% of the market it could be a great bike. Frame looks sturdy as heck. I plan to put it in a suitcase so I'll be removing the swingarm to do that. I'll report later on that. So far no regrets but time will tell. If you never saw a higher end folder, most people would be quite happy with it. As I said, 80% Dahon quality at about half the price or less. You can make your own judgement. I am not here to proclaim it's the world's greatest folder but it seems to be a darn good value.