The 458 is more "up and over" than twisty. There's a mountain pass to go up/down which is very steep in parts and quite a sharp bend on the way down. If your heading off towards Welshpool at Mallwyd, there's a very tight bend over a bridge to watch for; it's fine towing, just approach slowly and hope there's no coaches coming the other way... Then there's quite a 'persistant' corner approaching Llanfair Caereinion; it seems to go on forever.
When I travel to/from Barmouth, I tend to go via Bala/A5, which avoids the mountains and is a little straighter, but nowhere near as pretty.
MM