Giles, I have never even seen a J class under sail let alone been on one bit I have sailed a Swan 43 around the Isle of Wight so I understand what you mean by the power in the rig. If the fore or aft stay gave way it would probably cause some real damage.
Those old S&S boats (like a J class) have such fine stern sections that downwind they can really get quite a roll on.
My concerns about the power are entirely related to our symmetrical spinnaker, which is a huge 150m2, with big shoulders, so a lot of power high up. Once it starts to blow a bit (more than about 15 kts), unless you have someone REALLY good on the helm, it can all start to roll and weave, and the prospect of a windward broach becomes quite a strong prospect.
That's what scares me.....