Well, many manufacturers certainly try to enhance their specifications by questionable means, but a bow sprit and rudder are clearly not part of the deck and shouldn't be represented as such. Length at waterline is a better indication because it represents the basic boat. Modern enhancements like plumb bows and reverse transom add waterline length to enhance performance, but may not add any interior space. Multi hulls have been very actively pursuing these and other enhancements, so the lines are becoming blurred. Some manufacturers even measure around the the gunwale to represent length.
I believe the best way to choose a boat for yourself is to first take an honest look at what you plan to use the boat for and what other principle parties (wife, kids etc.)expect it to be. There are so many designs out there because there are so many demands. You cannot have it all in one boat, so don't be swayed by claims that this boat has broken the engineering barriers and can do it all and better than any others. Stick with the basics. If you are going to be a serious racer, buy a serious race boat. If you and family plan to spend time together enjoying the many joys of being on the water, buy a comfortable cruiser.
I have owned boats of design from many opposing aspects. The trimarans I've owned were extraordinary performers, not just in speed but every measure of performance. My current 23 PH is anything but fast or points well to weather, but she is comfortable and accommodating. Each of these boats sail very well to their design purpose. The boats on the market today that try to capture all the best in one design, are actually just trying to capture inexperienced buyers, and usually those boats don't do anything well except sit at the dock.
In short, I don't think any of those numbers are more than just a guide and not a predictor of performance or owner satisfaction. I've owned 5 Com-Pacs, the first being a 16 new in 1979, in which I cut my teeth on Chesapeake Bay. That boat taught me a lot and never failed me. Every Com-Pac I've owned has been faithful to it's design purpose, so I have been happy with them all. Look at boats like Com-Pacs that stick to proven design and shy away from boats that try to represent a 30 foot boat that's trailable in 25 feet of length, the numbers just don't add up.
If you have the time and patience you can find much written material on naval design which will open your eyes to many mysteries of boat design.
P.S - had a very nice sail on Charlotte Harbor today. Nice to see you out there Vecktor Director Ryan, and a hello to whom ever was sailing that Horizon Cat with the Black hull. We kinda had an unofficial Com-Pac rendezvous. And a big thanks to the inventor of sailboats.