I know that road, it is very picturesque. But it does entail a further detour to the south via Builth Wells, which I agree is really lovely. But at this rate the OP won't get to Aber in a day, unless he starts out really early!If you want something really entertaining, try the Abergwesyn Pass ( ends in Tregaron ). Mostly single track road so not fast.
If there's been enough rain, the waterfall here is very picturesque -