My opinions on "hot to the back." Depending on the conditions, you are going to sweat at times. And it is just part of the sport, like many other sports. I don't worry about it. To me, sweat droping on my sunglasses is more of an irritant than a sweaty back.
Here is some feedback on packs I have used.
I have Dueter Futura Pro 42 and a REI Venturi, both have suspended mesh panels. Yes, they are cooler than other packs, but at a penalty - extra weight and moving the load pack a little bit from your center of gravity... not a lot, but it moves it. And like any other pack, you will still sweat in hot weather. Plus the construction pushes the mesh panels against the pack material and the space is awkward for packing stuff. And I mean really awkward. But I still have both packs and use them occassionaly for specific hiking... usually in temps over 100F where I am carrying mostly water.
The GG packs have an outside pocket to place your sleeping pad, so the pad rests against your back. I normally use a GG Nightlight pad with the bumps against my back. I supose this might provide a tiny bit of ventilation, but probably nothing the scientific folks here could measure. :)
The ULA packs are constructed for the sleeping pad to fit inside the pack, resting against the pack material that pushes against your back.
The ULA's in general are a itty bit heavier to similar GG packs, but are made of more robust materials and I would anticipate they will last longer with similar use.
So which pack? I have three criteria:
1. Pack that is comfortable to carry.
2. Pack that is easy to live out of (I don't see this mentioned a lot).
I had a ULA Conduit and an Ohm. I used the Conduit the most of the two. It was comfortable, but not extremely comfortable. For me it was hard to live out of, because I hated the short slanted side pockets. I could not carry 1L platys in them, without them falling out. It just drove me nuts.
When I was looking for a SUL pack for short trips, I was interested in the ULA Amp, but it had been discontinued and I could not find one. So I bought a GG Murmur. I love this pack, and it was so convenient to live out of. So based on this, I decided to get a GG Mariposa Plus hoping it would be able to replace both ULA packs. And this pack is wonderful to live out of, and also carries well. So I sold the ULA packs. Nothing wrong at all with the ULAs, my personal preference was I just liked the GG packs more.
And in warm and hot weather I have a sweaty back, and I just ignore it, not high on my list. It seemed like I sweated more with the ULA packs, but not enough to warrant any concern.