Second: BD will have a similar if not the same model on sale eventually. Not sure exactly how they order/acquire their frames, but they seem to do limited runs, and also advance purchases--order now for a decent price on a bike which won't ship until June, which gives them up-front money to place an order, maybe? But if something sells well, they will offer something comparable soon enough.
First: Effective top tube is one component of fit, helping a rider determine how far forward s/he will have to lean to reach the bars, depending of course on stem length and angle. There is another measurement, Reach, which is the distance from a line rising vertically from the bottom bracket to the ETT, to the top of the head tube. length of head tube also makes a difference---with a taller head tube the stem and bars start out higher and thus effectively closer to the seat.
Once you find a good position on a bike, you can set up different sized bikes (within reason) to those same dimensions by using different lengths and angles of stem, different shaped bars, and different saddle set-backs, to some degree.
As far as I understand it, the one measurement never to mess with is the distance the saddle is behind the bottom bracket---there is one size that is anatomically correct for you, which gives you max power and no knee damage, and the only way to adjust that is to shorten or lengthen your femurs.
If you are comfortable with a bike with a 54 cm ETT, you should be comfortable with a bike with a 55 cm ETT and a shorter stem, or bars with less reach, or under the stem spacers, or a higher-angle stem---or maybe just stretching a little more. Back in the day, I had some bikes set up differently: some were really racy and useless for more than 40 miles, some were really upright and great for ten-hour days touring, and some in the middle.
I plan my next bike to have slightly more saddle to-bar drop---the height difference between the horizontal line through the center of the saddle and the horizontal line through wherever you put you hands (I usually have my bar-tops parallel to the ground but not all do.) This will put a little more strain on my back but I am building the bike for days when I want to go fast and not mess around, when my legs feel good and i will be able to support the load with my thighs instead of my core.
Once you find what works, though, you probably want to stick close to that. You might get stronger or more flexible and be able to lengthen your stem or drop your bars some, or you might get old (like me) and go the other way, but once you find what works, you will look for bikes which you can set up that same way. (Possibly. How would I know what you will or would or should do? I am just some imaginary guy on the Internet.)