There is something definitely wrong with this rigger or the guy he has working for him. When I picked the boat up, I asked the assistant why he ran the backstay aft of the stern rail. He replied "Well, it touches the rail either way and that seemed like the best way to rig it.". Later, I noticed a 3/8" gap between the top of the chainplate and the stern, which led me to notice the backstay was bending over the top of the stern rail, essentially making it part of the rigging (with all the tension mentioned in the OP!)
The first pic shows the way the rigger had it. Not a great shot, but notice how the backstay deflects over the rail and you can only see half of the turn buckle. The second pic shows when I moved the backstay forward of the rail. I'm no rigger, but the second line is so much more natural and obvious, it's laughable! The backstay barely kisses the rail, and the chainplate is against the boat again.
Funny thing is...this rigger is well known, has a big shop, and I have no doubt he works on boats who's spreaders are worth more than my entire boat. I guess he just wasn't interested in my little project and delegated it to his assistant who apparently doesn't have much common sense.
This is not as serious as you (and others) may think.
Surprisingly low force is needed to move a string some little distances. Think about a guitar string, tensioned as it is, it is still easy to move it.
It really looks as the backstay should not touch the stern rail. So, yes it was not really correct mounted. But this doesn't make the stern rail to be a part of the rig, or to actually handle large forces.
(on my boat, ~40 ft, I deliberately let the aft stay rest on the stern rail in order to easier open the aft lockers ... I have no problems with this. None. ).
You are upset that the rigger has set your rig with high tension. I would not do that myself, as I am lazy - prefer to go on the soft side. But there is nothing wrong with doing so, even if it exceeds all strange recommendations.
There has been some interesting argumentation in this thread. Some of this is clearly home brewed thinking. Personally I mostly agree with Rick H who correctly identifies that all materials will have be elastic (to some degree) and thus works as a spring. the mast and rig acts as a system of springs, so does the hull (+ deck etc) and it is this combination that should tuned to something that works in the wind range targeted.
Some further comments:
1) even if you may be far from the breaking tension, the material (wire, whatever) may very well be deformed. Generally speaking, one should keep tension well below breaking point in order to avoid deformation, for wires - elongation.
2) High rig tension may damage the hull and expose the chainplates fastening for higher stress than really intended (rather common issue on older boats).
3) a wire is a spring in more than one way, firstly as the material in itself is somewhat flexible and secondly as the wire consists of thinner threads spunned around eachother, just like a traditional rope or a ... spring!
4) Have you ever thought about what happens when you tighten the back stay in a blow? The top of the mast is moved back and down, thus the side stays will be less tight, the mast will lean to the lee side - a way to spill wind which is good in the blow.
But, again, that means that some of the tension you put in the side stay is lost.
This is a good though example demonstrating that it is worthless to speculate on static settings. Rig shall be set during sail, alternatively the rigger should know what he is doing.