Zipps are definitely faster than conventional 32-spoke wheels. However, the difference really isn't all that much, and certainly rather insignificant if you're not racing (real or otherwise). To objectively assess this performance gain, may I direct you to Analytic cycling's breakaway analysis page.

Given a rider mass of 65kg (143lbs), a bicycle mass of 8kg (17.6lbs), and a rider output of 350W (i.e. significantly better than non-sandbagging Cat 5), on a flat section, aero wheels will only net the rider 14.6m or 1.2sec per 2km (that's 50ft in 1.25mi). For the OP's 15mi loop, the aero wheel effect will only account for about 600ft or 14.4sec. That is with 350W of power. Reducing the rider output to a more normal 250W, the difference will be more visible. But it's still at 14.6m or 1.4sec.

Totally eliminating the drag of both aero wheels (i.e. magical elven wheels) at 250W of rider output will net 66m or 6.1sec per 2km (218ft in 1.25mi). This is about 1/2mi in the 15mi loop. The OP stated that with the Zipps, he increased his average speed from high-17s (let's say 18) to 19.2mi/hr. That's a difference of nearly a mile (or 3min) between the two at the 15-mile mark. Obviously, even with Elven wheels, the difference cannot come solely from the wheels.

Thus, the substantial increase in average speed from high-17s into the 19s cannot be solely attributed to the fancy wheels. In fact, from this analysis, it is clear that the improved aerodynamics of the wheels only offer a small advantage.

Personally, I wouldn't mind a set of DV46 myself, even though I know they won't give me any dramatic speed increases, at least not $1800 worth. However, they're cool as h*ll, goes well with the other carbon bits, and the wife unit will certainly accept this better than another bike. Like what someone else stated, it's your money, so don't let anyone tell you how to spend it.