I currently own a Tamron 17-55 f/2.8 lens which I love. It is not labeled as a wide angle lens. Is it beneficial to purchase a lens explicitly labeled as a wide angle lens (like the Rokinon 16mm f/2.0)? Will it really offer me anything different than the Tamron can (besides the extra stop)?
You are asking two questions:
- Is the Tamron 17-50mm f/2.8 a wide angle lens?
- Does a specific wide angle lens have a benefit over the Tamron 17-50mm f/2.8?
I have two answers:
- The Tamron 17-50mm f/2.8 lens is a zoom lens and covers the range of a wide angle lens as well as a normal lens and depending on the definition you have, can also be considered a short telephoto lens.
- As you already noted, the lens you suggested does offer more light gathering abilities but the fact that it is designated and marketed as a wide angle lens does not itself give you any additional benefit. If if was a Brand X 17mm f/2.8 Wide Angle Lens, there is no reason to believe that it will provide any different images than your Tamron that is not sold as simply a wide angle lens.
For more information I would recommend this question: If a lens supports 18mm focal length, is that a wide angle lens?
The Tamron SP 17-50mm f/2.8 Di II is an APS-C only lens. That means it projects an image circle large enough for APS-C sensors that are roughly 24 x 16mm (Nikon. Petax) or just a tad smaller in the case of Canon. On a Canon APS-C camera it should provide a diagonal angle of view of around 78.5º
The Rokinon/Samyang/Bower/(whatever else it is being labeled as this week) 16mm f/2 ED AS is also an APS-C only lens. It offers a slightly wider diagonal angle of view at 79.5º.
In contrast, a 17mm lens with an image circle large enough to cover a 35mm film sized "full frame" sized sensor provides an angle of view of around 105º when used with a FF camera. During the film era an angle of view greater than about 85-90º was considered "wide angle." By this standard neither a 16mm nor 17mm lens provides a "wide angle" field of view when used on a APS-C camera.
That one is labeled as a wide angle lens by its marketers while the other is not has absolutely no effect on the angles of view provided by each lens. It's more a reflection of the time period when each was introduced. They're both rectilinear lenses and also present roughly the same projections of a 3-dimensional world onto a 2-dimensional imaging sensor.
At the time the Tamron SP AF 17-50mm f/2.8 XR Di II was introduced in 2006 almost all digital SLR cameras had APS-C sized sensors. Nikon, Pentax, and Sony had yet to even offer a single digital SLR model with a full frame 35mm film sized sensor. In that environment, a 17-50mm digital lens (i.e. one for APS-C digital cameras) was not really considered wide angle since the field of view using such a lens on an APS-C camera is similar to using a 26-75mm lens or 28-80mm lens on a traditional 35mm film camera. The 28-70mm or 28-80mm zoom used with 35mm film has traditionally been referred to as a medium zoom.
Only with the emergence of many full frame camera models beyond a very few top pro bodies has the use of wide angle once again been used to describe lenses in the 16-18mm focal length range intended for use on digital cameras (which no longer assumes an APS-C imaging sensor format). Apparently this is even the case with some recently introduced APS-C only lenses in the 16-18mm range at their widest angle of view.
Whether a lens is explicitly marked as a wide-angle lens or not is kind of irrelevant. What is important in terms of 'angle of view' is the focal length (or focal range in the case of a zoom lens).
Comparing your 17-55mm lens with a 16mm lens, the 16mm lens has a focal length 1mm shorter than your zoom (at its widest). Technically, a focal length 1mm shorter will produce a wider angle of view, but a difference of just 1mm is too small to bring any meaningful benefit.
Take a look at this page for a graphical representation:
17mm is definitely wide angle, regardless of whether it is marketed as a wide angle lens or not. 16mm will give you just slightly a wider angle than you can get at 17mm.