Espruino WiFi: WiFi tweaking possible?

Yes, absolutely! The WiFi module on Espruino WiFi is a standard ESP8266 running AT command firmware - and the docs for it are here: https://www.espressif.com/sites/default/­files/documentation/4a-esp8266_at_instru­ction_set_en.pdf

The best way to reduce power is to stay disconnected from WiFi with the ESP8266 off for as long as possible, but there are some AT commands that might help. AT+RFPOWER and maybe AT+RFVDD look promising, but I'm not sure if there are any others.

To send the command, try:

wifi.at.cmd("AT+RFPOWER=50\r\n", 1000/*a timeout in ms*/, function(d) {
  if (d=="OK") console.log("All Ok.");
  else console.log("Error",d);
});