You would be brewing something close to a barley wine. You will want to use a different strain of yeast that is alcohol tolerant and the beer will take a long time to be ready. Maybe a year or more.
I say give it a shot!
The better alternative is to use the same amount of water, but twice the ingredients
Are you an AG or extract brewer? With AG, cutting back on the amount of water will result in medicore efficiency. With extract, you need to be careful about hops utilization when you have a really, really concentrated wort (assuming partial boils). As Rich noted, some yeasts are designed for very high-gravity worts; use one of those instead of something like a champagne yeast. And yeah, look at some barleywine and Imperial IPA recipes for suggestions.
For REALLY big beers, you may even - GASP! - want to use a little corn sugar (a pound or so, 10% of the total sugars) so that it doesn't end up way too thick and sweet.
I made a Brewer's Best Russian Imperial Stout with only 4 gallons once. I also added some corn sugar and lactose to the recipe. The shortage wasn't on purpose though, just drunken carelessness. :cross: It was definitely thick and was most excellent after about 3 months aging. Some of my friends compared those to an unmodified BBRIS batch and most of them preferred the thicker batch.