To my comprehension a BALUN/UNUN is an RF Transformer. If this is correct and since a BALUN/UNUN is part of the antenna system ... how much power does one expect to lose there?
Specifically, how much sense does it make to compromise ERP in lieu of band choice when a rig is capable of perhaps 15W at best?
A balun is any device for converting between balanced and unbalanced systems. Not all of them are transformers.
Losses depend on the kind of balun. On HF, it's common to use a common-mode choke as a balun. Losses on this type of balun are quite low: just the additional loss of the transmission line wound around the ferrite. It will depend on your transmission line and frequency, but this is likely less than 0.1 dB. If you want a more specific number, the spec sheet for your feedline will have a loss specification.
It depends. A choke and/or transformer between the feedline and the antenna changes the ratio of currents flowing on each, and thus where the EM radiation and thermal losses occurs. In some situations (SOTA QRP with a poor ground, short low antenna, low shock hazzards), additional EM radiation caused by unbalanced currents over a longer feedline might add up to send a better signal towards some destination. But with a better antenna (in terms of radiation resistance and directional gain) or a more lossy feedline, less likely so.
