I was really worried about this stuff when I first started using slings. I've gotten good at learning my body's tolerance to cold, and what my clothes are capable of. I rarely add or take off layers in the tree now, with exception of a heavier jacket once I'm settled in.
A couple of notes on the topic:
- Bibs or pants that zip to the hip are the shiz. Especially if they have double zippers for both directions. It allows me to put bibs on at base of tree, climb, and unzip to cool down if I get hot climbing. Bibs that zip this way may solve your dilemma of needing to modify something, though I haven't had a need to try it.
- Being used to my climbing methods, and going very very slow, have kept me from sweating with heavier layers on. This gets said a lot on here, but can't be stressed enough in my opinion. Some folks like being up the tree in three minutes. It's nice if you're running late. Though it's difficult to quantify, I feel my sightings have gone up due to quiet, deliberate, slow setup going up the tree. Mostly, it feels nice to be relaxed and calm slipping up the tree trusting in what I'm doing.
- It's probably too late to change if you're experiencing this now, but you got the wrong saddle if you can't layer up without it becoming to small for ya. Too big is better than too small. Having said that, I'm borderline needing a size 2. My size 1 kestrel fits swimmingly with layers to get me into the teens. I suppose if you're hunting single digit weather I could be wrong on this account. But if you're on the fence on a size - go up. You can always sell it for ten bucks less on here if you're wrong...
- I have put on and off pants in the tree with the kestrel. I just went one leg strap at a time, then waist strap. But I had a nice roomy platform to make that maneuver! Probably wouldn't be the easiest thing in the world on any sort of steps or small platform.