I wouldn't hike in a fleece and windshirt unless it was below freezing. I wouldn't hike in a Nano Puff as it won't breathe well enough.
I often hike in a base layer and windshirt and add the mid layer fleece (R1 or Power Stretch level) for rest or camp use. If it is colder than that, I would graduate to a 100g fill garment like a First Ascent Igniter or Patagonia Micro Puff.
IMHO, the 60g garments like the Nano Puff and Mont Bell Thermawrap don't cut it. They are too close to duplicating a windshirt, which I am already packing, and I can use the windshirt plus the fleece mid layer in many combinations.
You can add up the weights and volume, but it's not a spreadsheet decision. It has to work on the side of a cold wet mountain.
My large size Nano Puff (that fits like an XL) weighs 12.6oz
Other items for comparison, all mens XL:
EMS Power Stretch Hoody 13.8oz
R1 jacket 14.2oz
Marmot Power Stretch vest 7.8oz
MEC Power Dry (like R1) vest 7oz
REI Revelcloud vest 10oz
Military Gen III fleece jacket (like R3) 19.4oz
Generic Polartec 200 jacket 18oz
Going back to my Houdini/R1 combo:
* It weighs 6oz more than the Nano Puff
* I can wear each separately or together
* I can wear the fleece with my rain shell and still have some breathability (try that with a Nano Puff)
* The R1 is excellent for sleep
* Adding a fleece at a rest stop will allow me to continue to shed excess moisture/perspiration where a Nano Puff would trap it.
* Wet weather performance of the R1 is excellent, not only with the ability to retain some insulating quailies when wet, but also the ability to transport perspiration from base layer and out to wind or rain shell.
BTW, I would be using the Power Stretch hoodie more often, with even more coverage at less weight than the R1. I feel that the two fabrics are close as far as warmth.
Bulk issues are mentioned with fleece. The lighter versions are more compressible than the classic 200W stuff. An R1 or Powerstretch jacket will pack in around the other items in the top of my pack, or carried under the lid where I can access it quickly for a rest stop.
Vests make for excellent summer insulation alternatives or even cooler with long sleeve base layer tops. If you want to save weight and volume, vests provide good core warmth at minimal bulk.
Note the REI Revelcloud vest at just 10oz. It is more breathable than the Nano Puff and from everything I can see, it has more loft as well ( the fill weight is not published). If you must go with light/thin lofted insulation, I think this makes a good or better alternative to the Nano Puff when used with a wind shell and long sleeve base layer.
Ultimately, I think these thin shelled/lofted garments are a failure in cold wet, high humidity environments. They won't transfer moisture and can't be worn while active. Layering a shell and light fleece mid layer can be far more versatile and even warmer at the cost of a few more additionsl ounces and a bit more bulk.
I think the stumbling block comes in thinking of the lighter lofted garments as "demi" versions of the classic belay jackets, which they are not as they aren't used in the same conditions, notably sub freezing and dryer.