You don't need an "extended" test ride. Anyone who tells you that, quite possibly, has never bought a bike before or has wasted their life weighing up the differences!
I have a tarmac and Madone - each feels less comfortable than the other, slower than the other, the wrong size etc DEPENDING ON WHICH ONE I RIDE THE MOST! All bikes feel different - the SL2 is going to be a lot stiffer than what you currently ride, with the beefed up BB Shell and 1.5 inch and bolstered head tube, it is going to feel stiff. Carbon has changed significantly since 1995. The bike should feel naturally quicker. Its very unlikely you will feel the Compliance benefits of the Roubaix however, certainly not on any shorter ride. When I first went for a ride, i did notice the vibration differences on slightly rougher roads.
Biggest advantage for yourself with a Roubaix is going to be the sizing and geometry. I don't want to presume things in an agist mannor! However, as you age you will probably find that the increased head tube size and slightly shorter top tube will be significantly more comfortable. As a bigger rider all Specialized geometries should work for you, that is what i find.
If you have the money for the S-works, go for the S-works. If you are stretched for cash, do look at the other models. As mentioned previously, the variances are not major. However, the S-works is by far the best. Although people have mentioned it comes with R-SYS - this is not the case Nearly every Roubaix SL2 has come with Kysirium SL's.
As for not having Carbon wheels, I'm not sure where that comes from either. Yes, don't get a set of Lightweights or any uber light wheels (not for strenght, but because of flex under power) but there are so many other carbon wheels that will be fine. I have a 2 sets of Carbon wheels and don't weight much less that 200lbs - I snapped my first Madone frame but the wheels are fine - never had a problem.