Thanks for the calculation, my trysail is about 90 sq ft, so about 20% of the main. I used your formula and got just under 1400 lbs at 60 knots and just under 2000 lbs at 70 knots. So considering that I would use it as a turning block that would make the load somewhere between 2800 and 4000 lbs. The 5000 lbs is the safe working load of the block - with the breaking strength at twice that I would assume. Not sure what the 8500 lbs block would be used for - what a beast it would be.
As for hooking up the sheets, I would have to use the cleats since I don't have a slotted toe rail - just the fancy and not very functional teak toe rail (good for keeping your feet onboard though). I have a couple of really big snatch blogs but if the shite hits the fan I have them tasked for the parachute anchor set up like the Pardeys suggest with a bridle and I might well heave-to with the trysail still up and sheeted in tightish.
Back in the water in Grenada - with new main and #2 and cockpit canvas (Santa came early). Will spend the winter and early spring in the Caribbean and then head to Bermuda and the northeast US. Still trying to decide if we will bring the boat to Canada, either in 2015 or 2016.