Thanks for clarifying your question a little bit. I am glad to hear you regularly top off your barrels, it’s a practice all of us need to do. Alcohol and water definitely do evaporate out of barrels (along with small amounts of other volatile aroma constituents of wine) and the resulting headspace does need to get refilled as this occurs. I find that topping monthly, usually replacing between 1⁄4 to 1 gallon every time, is enough to keep my wine sound. The given amount of wine you lose in the “angel’s share” as you refer to it above will change depending on the humidity and temperature of where you store the barrels, as well as the age and stage of your wine. I always lose more volume when my wine is very young and going through ML as it’s so gassy. The CO2 blows off and you lose volume as the wine settles down. And I lose less when the wine is sulfured and simply aging, anywhere after about four months of age. Don’t forget that you also lose volume due to lees loss every time you rack off of lees of course, too.
I have to admit, I’ve never seen an industry or academic study that looked at the concentration of non-evaporating (i.e. color, tannin etc.) compounds in wine over time due to water and alcohol loss being replaced by wine. If you look at it from a physics and chemistry point of view, it would seem that some kind of concentration could be happening. However, as the non-evaporating flavor and texture compounds in wine form less than 1% of the total wine volume (around 0.01–0.5%, most of the volume of water and ethanol) I would imagine that any possible effect would be so slight that the human palate would not detect it. And if we can’t tell the difference, then why do more topping than you normally would?
Remember, every time you open your barrel, you introduce air and potentially some undesirable spoilage organisms. I’m happy only opening up my barrels once a month to top them up. Though you could presumably speed up evaporative loss by warming up your cellar or decreasing the humidity, I’m not sure I would do that either. Warm temperatures, over time, can encourage spoilage yeast and bacteria to grow while dry air will wreak havoc on any barrels that you might have empty and could even dry the outside of full barrels unevenly, possibly leading to barrel integrity problems later on. I’m not sure that any incremental benefit in wine color and flavor concentration you might get (emphasis on might get) would justify tweaking with the rate that wine evaporates from your barrels and has to be replaced by . . . more wine!