You can use the full amount, but as Jag said, the trouble might outweigh the benefits. Some people argue that full batch brewing (as opposed to "dilute to volume") is better quality, but that's typically in terms of all-grain brewing, not extract. If you do go the full volume route, as Jag mentioned, you'll need a practical method of quickly chilling that much wort (unless you're doing no-chill brewing - different discussion).
Off topic, but one thing you can immediately do to improve the quality of your extract batches is to add 5-10% of your extract for the full boil duration (for hop utilization purposes), then add the remaining extract at the last 10min of the boil. This prevents wort darkening and flavor changes. This process will improve your extract brews more than using full volumes of water.