Check the frame manufacturers website and see exactly how the 55 and 57.5 bikes are different.

I have 4 bikes. A "56", a "57" a "60" and a "61." I'm probably a 58.

I'm comfortable on all of four bikes. The seat tube angle is the same on all of them, so saddle positioning is simply a matter of adjusting the seat so that it is the proper distance from the bottom bracket. The only difference is that the smaller bikes have a bit more seatpost showing.

Bar setup is a bit more tricky. The two small bikes have non-road bar setups. One has bullhorns and one has moustache bars. The two large bikes both have drop bars. The 61 actually has a 59cm top tube. But because the bars I'm using with it have a longer reach, I'm actually using a shorter stem than the bike with a 60cm top tube.

The bars on both small bikes bikes are about 3" below the saddle, which is mid point between the tops and drops of the bars on the large bikes. To comfortably run drop bars on the small bikes, I'd need a very long and tall stem.

There are many variables besides frame size that affect bike fit. The reach of the bars, how the brake hoods are positioned, and the angle of the bars all affect the upper body. The length of a saddle and the size of one's feet affect the lower half.

And finally, in 40 years of knowing how to ride a bike, I've never needed that 1-2 inches of top tube clearance they say you need to keep from crashing your nads on the bar.