Shifter size query

If you spend much time riding on the top of your bars you might find a shorter stem will be uncomfortable so a pair of bars with a shorter reach may be a better option: the same reach to the top of the bars but reduced reach to the shifters to compensate for the extra length.

I doubt the extra length in the front of the hoods would make a massive difference, if any, to position. I'm fairly sure quite a lot of work went into the design of the road hydro groupsets to make the positions as similar as possible.

If it is a problem, you could always take a frame size down on the new bike and consider some bars with a different shape to the drops to compensate

They are indeed a little longer; I have ST-5700 105 shifters like you and I can tell the difference when I tried holding the ST-RS505s on a demo bike.

However, I think you can get by with the same stem length. Most of the added length on the ST-RS505 levers is in the "bulbs" at the end, I feel. As many people have said, although the bulbous ends of the hoods are unsightly, they make for good handholds while riding with elbows bent, on the hoods, in an aero tuck.

Best way to be sure is to test-ride, of course, but I hope this sheds some light.