i have very limited experience but I've drawn the same conclusion from all of my little slings.You can't powerfeed a sling from what I've gathered. It will just go into premolt sooner and stop eating for a longer period of time. You can't speed up the time that is needed to form a new exoskeleton underneath the old one. The concept of powerfeeding came drifting from the herp-hobby but is not applicable on Theraphosidae.
When a sling is full and has all the resources that it needs, it will simply stop eating.