It will be less efficient, yes. But it is still a net positive. If it is cheaper to build it there, you can just add more power to compensate for that loss in efficiency.
It will not be a 100% loss. For reference: the power loss of the transmission in the US is on average 5%. Let’s say it is double that, it is still very little in the grand scheme.
For opensource maps, it will be open street map. If it is outdated for you area, open a few issues, add a few things yourself, and you will notice that others will join to help.