A straight razor (1/1 hollow ground) has a very specific blade profile, which lends the blade rigidity while maintaining a good amount of flex. It has also a very soft steel. A normal edge geometry and blade steel is too rigid to give a comfortable shave, but it will cut the hair just as well. It's is really not so much about the sharpness as about the comfort. The new safty razors all try to imitate the natural flex that a good straight razor has "build in" by balancing the the blades on springs, etc.
In terms of sharpness, it is hard to beat a regular BIG razor. But at some point it seems to me that sharpness stops being much of a factor for a good shave. Cleanlyness of the edge seems much more important. I have noticed that the comfort of a shave with a straight razor improves substantially during the first 10 or so stroppings after sharpening even though the measured pushcutting sharpness does not improve significantly. I think the edge simply gets more polished and cleaner, which is also why I think a straight razor is more comfortable (to me at least) than a BIG disposable razor.