Toby, on high tensile I usually use two crimp fittings. I can't think of a place where I've used Gripples on my high tensile. Even where a tree comes down on high tensile the wire doesn't break. Cut the tree off it and the wire pops up. I replace the clips or staples and maybe take a part of a turn on the nearest tightener and it's usually good to go.

The 200 ksi (high strength) HT wire is so hard the Gripple rollers may not be able to bite into it.  With a loop secured by 2 crimp sleeves I've never had an HT joint come appart.  I do think the key to success with HT is having a spring and tightener near the middle of each long straight run. The spring lets you tighten it to a known spec tension and compensates for temperature changes and impacts.

On barb wire I've had very good luck with the Gripples, both for new construction and repairs.  The key is to get the right Gripple for the wire size. Barb wire being flat ( 2 x 12.5 ga) has a special Gripple. 

The only time I've ever had trouble with a Gripple is when I've tried to reuse one that has been in the fence several years. I need to resist the temptation to keep used ones when I pull out a fence and just throw them away. It's not worth it trying to reuse old ones.