What difference in texture and taste does using hot water make to pastry? How would this compare with a standard recipe (flour, butter, salt, egg, a little cold water)?
For shortcrust and puff pastries, the aim is a light pastry. This needs the flour to stay well mixed with the fat, and as little gluten development as possible. Therefore we add as little water as possible, and keep the pastry cold. Using hot water would just melt the fat. So I would not recommend it.
On the other hand, hot water pastries use a larger quantity of boiling water. Here it is deliberate that the fat melts. The result is a much stickier and malleable pastry. Typically it is used for savoury pies where a robust cover is essential.