I notice that the geometries google conjures up for the sempre pro and 928 as given by bianchi are almost identical, within a mm here and there, with the STA the same. The good thing though on the sempre pro is they list stack and reach, so you can probably save yourself the math and use the sempre pro charts.

However, I don't know how to tell someone what stack or reach they need by their height and inseam. Reach depends on how much seat setback you like (for balance) and stack depends among other things on how far you are able and willing to bend.

Compared to the quick speed 2 though, that 55 looks like about 5cm less stack (lower handlebars) and about 2 cm less reach, but check the tables yourself, don't take my word for it. My non-pro guess is you'd fit something in between those two.

I'm guessing that shorter reach is ok and maybe even good depending on your setback preference. At most it means you'll need a 2cm longer stem. The stack might be pretty aggressive, but only you can know. I also notice though that you can go all the way up to a 61 frame (based on sempre specs) and the reach only goes up 1cm, still a cm less than your QS, but the stack just keeps getting higher. The 61 actually seems to claim a stack about the same as your QS while still having a reach a cm shorter.

I didn't take any care about year models though, so you should double check it all.