I didn't record the daytime temperatures but I recall quite a mixture, probably not dissimilar to northern Scotland at the same time of year. A lot depended on whether the wind blew or the sun shone. My daytime kit didn't struggle, it was the evenings/nights that caught me out a little. I'd probably take zip-offs as there's certainly a chance it'll be warm enough to use them.
I went to Trollheimen, only just north of those areas, in late Aug / early Sept a few years ago. I'd taken a PHD minimus bag (rated +5c although I can use it a bit lower than that) and I found myself on the edge of comfort. The first couple of nights it dropped to -3c and there were light snow showers. There was also a persistent cold wind, so it didn't help that my tent only had a mesh inner. The view of the few Norwegians I met was that summer had ended a few weeks ago and winter would arrive shortly. Having taken my summer backpacking kit which handles UK conditions or the likes of the Pyrenees, I'd certainly be upgrading it to my autumn choices if I went back at that time of year - probably to a Minim 300 (0c rated) and a down jacket like my Minim Ultra, and definitely a full 2-skin tent.
They're all good, but don't forget the long horseshoe... up Moel Eilio, along the ridge and over Moel Cynghorion (great views in to the cliffs of Cloggy from there) and join the Ranger path up to the summit. It tends to necessitate a return down the Llanberis path for transport reasons (but nearly deserted late on a summer evening that isn't so bad). I reckon if you could sort the transport and descend via the Rhyd Ddu or the South Ridge it might be even better..
1. Park by the old bridge outside Llanelltyd and go up through the woods W of the village to reach the New Precipice Walk. Use that as your access to and from the obvious horseshoe to Y Garn (629m). Great views over to Cadair, along the Mawddach estuary and into the Rhinogs.
2. On the S side of the Mawddach estuary, park up by Llynnau Cregennen and take the footpath E to Kings YH, then climb up to the campsite and the Pony Path to Cadair. But when you reach the col, don't turn E for the summit, head W along the superb grassy ridge towards the sea. It stays around 600m much of the way. After Trawsfynydd descend to the forest and the old track leading NE back towards your starting point. You can see some of the terrain on the early and later stages of this trip report.
3. The southern end of the Rhinog ridge - a bit higher, reaching 750m on Diffws, and sharing some of the ridge with Marcus' suggestion. You can park a car by the phone box at Llechfraith. There are several possible ways up Diffws depending on your preference. There's the track up the forested valley to the east that runs N or the ridge over Mynydd cwm Mynach, both sharing the final quite steep, rough pull up to the summit. Or the gentler ridge of Braich, which could otherwise be your descent. I've never tried the ridge directly up Craig Aderyn but I think it should go. A descent can follow the ridge all the way W over Llawlech to the Bwlch and then a good track leads back to Llechfraith.