After using Stirling Creamery's 84% butter in croissants, puff pastry, shortbread, pie crusts and slathered on fresh bread, I can honestly say this is an excellent product. Unlike regular butter which is about 82% butterfat, Stirling's butter is much more malleable right out of the fridge. If you've ever tried to roll out cold butter in a laminated dough, you'll be familiar with butter breakage in the dough and water seepage in resting dough that contains any sugar. This was not an issue with 84% butter. It rolled out easily, without breaking and was also much easier to rub into flour when making short pastry.
Laminated doughs baked up with identical rise when I did comparison side-by-side bakes. Surprisingly, the lower butterfat pastries tasted greasy when tasted beside the 84% ones. The 84's also had a sweeter and lighter flavour.
Shortcrust doughs were more crisp-tender, also sweeter and not greasy compared to the standard butter.
On fresh bread, the 84 butter was, hands down better in flavour and texture.
The downside is that the butter is more expensive, at least in the small stores I have been to, and the price will vary depending on who sells it. The 84% is also not as widely available.
IMO, if you're going to spend the time and effort in baking your own pastries, this is an attractive option to produce a superior good. If the market puts this type of product in demand we may see more producers making high fat butter available. I'll be keeping my fingers crossed.