Ok, my thoughts in general as I've been making/using soft shackles for 5 years and been in rigging/Chandlery for 20!!
Firstly I think you can't go past bowlines for cruising. On race yachts soft shackles are great for quick changes and repeating settings but because your sheets sit on the winch and blocks at the same place all the time, you'll go through sheets pretty quickly.
A bowling means you can end for end your sheets and as you will never tie the bowline exactly the same, you'll get far longer out if a set of sheets.
Now on soft shackles...a well made soft shackle is 1.5-2 times stronger than the dyneema it's made from, so a 4mm shackle should be at least 3000kg break and a 5mm around 5000kg!!
Now to give you an idea on loads, I have put a Harken 40st winch on a test bed with load cell and in a perfect winding body position with a double handed winch handle, I could get 1500kg max load.
So basically you could use a 4mm soft shackle on any halyard or sheet on any boat under 35ft, a 5mm would be more than enough for the jib sheets on Shaggys Pogo!!
Here's some pics of sheets I made for the Inglis 58 down here.
All 12mm spectra, jib sheets have 8mm soft shackle and kite sheets have 7mm ones!!
Both have velcro keepers at the owners request but I have never had a soft shackle come undone!!