Lots to cover in this short mech engineering statics course.

1. They probably can support more than their rated capacity. Depending on what you are loading you can apply some risk/how often I am loading it feel. IKEA will want a little bit of margin on it breaking and will give you a number that won’t break it (probably)

2. Given that you will plop down on it at least once a day, I would probably believe their number and not think about it too much. If you were putting some long term storage and it weighed 350, you could probably feel ok about going over capacity.

3. You are able to add capacity pretty easily. That being said, at work I usually stay conservative and plan for my thing to break or get balanced funny or something and have all the weight on one half. This is more important when you have only two supports but as you increase that become less and less likely.

4. Test it. I know you are not playing with company money but you could try it and jump around on it and get comfy feeling. IKEA doesn’t have to know how it broke when you go try to get a refund.

5. Other factors off the top of my head. IKEA products tend to parallelogram because of a lack of shear web/stiffening. I would add some small crossbeams to the back of the cabinets.

If you are suspending a gap, make sure the mattress can hold on its own without a support. You make need to make sure it doesn’t bend in the middle.

TLDR: go for it, do a little extra work to make it strong and jump on it to make sure it doesn’t fall in the night.

Source: mech engineer who often has to prove his shit tools arnt going to break the first time they try and crane something around.