In reply to SethChili:
I walked it in May in boots, Salamon Quests, or something. They were fine, but spare socks are a boon for when your feet get sweaty, to avoid blisters. I did it north to south and changed after the Carnedds. Last year, we tried it from the south and failed before we even got to the start! In the same boots, my feet were soaking, in the pouring rain, within an hour of leaving Pen y Pass. When I did the northern two sections a couple of days later, the several boggy sections, particularly on the lower descent parts of Elidir Fawr were so bad that a soaking was unavoidable. Take spare socks!
I did the northern half of the GR20 in Inov8s a couple of years ago. It was dry, rocky and I was really happy with the shoes, bar the odd clonk from pointy rocks. If I had a pair of Inov8s now, in the dry, and wanted to go fast over the Welsh 3000s, I'd wear them. And take spare socks.
I wore road shoes for the Welsh 3000s 25 years ago, again north to south, had wet feet for hours and hours, which became white, very puffy and painful feet by the time I got to Snowdon. Should have taken spare socks!
This time, we bivvied at the lake under Drum at the north end then set off at 4am for Foel Fras. That was good. We didn't leave stashes of footwear along the route, but did leave water under rocks in the Ogwen valley, for a refill as we walked along it before going up Tryffan.
The day after, I retrieved the tent at the northern end and the water bottles from the Ogwen.
Good Luck!