I haven't done this yet because I want good baseline measurements. Doing a little homework on it I've found there is potential. Using a boltzmann's law calculator and info from emissivity tables there can be a gain of about 360 watts emitted from 1sq meter, about the average sized 2-3 ton outdoor coil. The gain might be better because I believe the calculation is a flat plate vs a 1" thick finned coil.
I'm using .4 emissivity for oxidized aluminum and 1 for optic black. Temp of 120F converted to 321 Kelvin. Surface area 1 sq meter. 240 watts aluminum vs 602 watts pure black.
Free Stefan Boltzmann Law Calculator Online | Calculator@TutorVista.com
Table of Emissivity in the IR Spectrum
I will be using rustoleum high heat bbq flat black which wont be '1' more like upper .9but 3M black velvet coating 9560 series optical black rates as 1 for anyone looking.