A very long lens in Snowdonia, especially in summer, will likely run into issues with atmospheric haze dropping contrast. A westerly wind just after rain is the best option for distant clarity. An easterly can be awful as it brings particulate haze over from industrial England. The longest fl I tend to carry in the mountains is 150 mm ffe, though having said that I was using 270 mm ffe the other (non-hazy) day. Also if it's as windy as it often is, a long lens can be hard to hand-hold. On Snowdon itself, I'd say (but this is very subjective) going up from the north a long lens is less use than if you go up from the south or east. .