Here's the deal, for most measurement rules, sail area is measured as a simple triangle, typically the length of the luff x the length from the mast to the clew measurement band (depending on the rule sometimes measured along the boom, sometimes measured perpendicular to the luff) divided by two.
With regards to your question about whether the area contained in extra girth does really help?. Yes and typically not much.... upwind, roach gives very little advantage, but reaching and running the extra sail area can increase speed in light to moderate winds. If not measured and taken to extremes it could provide an unfair, unmeasured advantage.
To keep boats from taking unfair advantage of adding roach, under almost all rules there is a standard amount of roach that is permitted. This is usually defined as maximum permitted girths (measured perpendicular to the luff at specified points) and which are typically specified as percentages of the foot dimension. If the measured girths exceed those permitted percentages, then the boat gets a penalty but in some rating rules and classes, and in most one-design classes the sail cannot be used.
In many rules, these girths are found quite simply by folding the sail from head to foot, so that the sail is folded with the head laid over the tack. This produces a fold and the length of that fold is the Midpoint girth effectively the girth at the middle of the luff (MP/2).
With the midpoint girth held steady, the sail is then folded again so that the head now touches the mid-point fold and the new fold is measured at the new fold producing the quarter point girth (MP/4). Lastly it folded again so that the head now touches the quarter point girth and the new fold formed is measured producing the eighth length girth (MP/8). This protocol is illustrated in the Northwest Region PHRF site. NEW PHRF-NW PROTOCOL MEASUREMENT DATA
Some classes literally restrict the sail area and in those cases, I don't know if you have seen this: http://www.sailing.org/tools/documents/GuideSailArea-
.pdf but it is from the International Sailing Federation and it descibes another set of methods for sail measuring, which may not be consistent with specific measurement rules but certainly would produce a very acurate sail area.
It should be noted that each one design class establishes its own measrement method and so if you are measuring boats of a one design class intending to race one design, you need to find the rules for that class.
In any case, there is no black magic or black art to it. Measuring sails is very straight forward, pretty quick, and quite simple.