Cyberforest.net's hummingbird page notes: "Don't make the mix any stronger [than 4:1]. Too much sugar is bad for the hummingbirds."
I doubt you're doing great harm since late-season hummers will be transient visitors & won't eat often of the unhealthy concentrate. You'd hurt them more giving them too concentrate a nectar when they are nesting, as they'd be using nearby feeders too regularly during that time. But overly sugared nectars can be harmful at any time if too often encountered, & in no case does increasing the ratio strengthen them for their winter journey. Here's an overview of why it's bad for their health:
Sugar water may supplement hummingbird diet but if they are so fond of sugar water that they neglect their NATURAL diet, they will become deficient in scores of minute but essential nutrients. Too much sugar might actually hamper their search for the more appropriate natural nectars. Ideally sugar-water feeders are not the only things the hummers find in a garden which will be full of trumpet-shaped spring & summer flowers, or for summer & autumn sage-blossoms, especially in reds, either planted in the garden or set about in pots. Providing potted plants in autumn bloom would be much more an energy-boost than more concentrated sugarwater for their autumn/winter journey.
Some hummers actually reject over-sugared artificial nectars, & good thing they do since it is unhealthy for them. As the Suite101 hummingbird page says, "Do not alter the proportions, as hummers will turn away nectar that has too much sugar."
But other hummers will favor feeders with the most sugar in the water & they will come from greater distances from their nests to feed, overlooking more healthful flower-sources along the way, knowing where they can get the bigger but mineral-deficient sugar load. These hummers become sickly & neglect their nests are are away from their nests too long at a stretch.
Those hummingbirds fed too much sugar can develop calcium deficiencies, muscular weakness, & bone malformations similar to rickets. Their eggs are apt to be soft-shelled & will not hatch.
Some hummingbirds appear to show signs of illness resembling drunkenness when feeders contain too much sugar.
Over sugaring their nectar also encourages rapid bacterial growth in the feeders & can also attract bees, wasps, & bee-flies.
Too much water is less harmful than too much sugar, though both can be harmful, as too little sugar could cause some hummers to not get their daily caloric requirement.
Some hummer fans claim their higher concentrates "fatten" the birds for their winter trip. I doubt that. But it certainly would not help them get to their winter homes if it were true they were getting fat!
I have planted for hummingbirds & I do not supplement their diet artificially. I have blooms for them persistently from spring through most of autumn, & in our zone the hummers are usually gone before the autumn-flowering sages & asters & Hibiscus moscheutos are done, so there's never a time when they cannot find something that is a favorite. It might be possible to make the hummers linger a bit longer if I were to hang the garden with feeders too, but I seem to have plenty of visits from the little things just by planting the flowers they like best, assuring them the very finest/ideal/natural nectars with all the required nutrients.
-paggers