I respectfully disagree with the philosophy that only distilled water should be used in extract batches, for the very same reason BrewinHard gave about using Spring Water.
The reason I think this is that the extract producers are producing a general purpose product for use in a wide variety of beers. They could not possibly know whether you will use this extract in a pilsner, an IPA, a sour, or any other style. We will not know exactly what the mineral profile was when they made it, but I seriously doubt that it was the right mineral profile for that Dortmunder Export, or that IPA, or that Schwarzbier, or that Berliner Weiss, or that RIS, that you've been itching to brew.
Putting myself into the head of the maltsters, what I would expect is that there was just enough mineral for them to convert the grains, and that's about it. By aiming low, they can "get out of the way" and give you a product that will tend to produce a "layup 5-iron shot down the fairway" decent beer every time, but you'll probably never be able to "hit the green for an eagle" right out of the box. Another thought is that at worst, they don't even take our mineral contributions into consideration; they just use their local water plussed up with just enough phosporic acid, calcium chloride and/or gypsum to get it to convert. Then who knows what style that would match?
My suggestion to the OP is to top up with water treated to be closer to the target water profile of the style you are shooting for, but back off a third or so to account for some unknown level of mineral content you get from your extract. Unfortunately, there is simply no way to get any closer. The best thing we can do is try it, and tweak it until you dial it in exactly how you want it to taste.
FWIW, I know that back in my extract days, my beers improved a lot once I began adding minerals to my water. I live in western WA state at the foothills of the Cascade Mts. My water profile is very close to Plzen, or just a touch above distilled. The difference was very noticeable, and it rang true for most styles. I just looked at the water profile target, figured out what I needed to add to hit that profile (using Promash), backed it off a little, and went with that. If it didn't taste right, I kept trying till it fit, but that was pretty rare. The amounts of minerals we are talking here are small and any quantity errors tended to fall below my perception threshold.
Most homebrewers progress from Extract to AG because of issues like this. We steep specialty grains rather than use amber or dark extract (What exactly is in that stuff? Roast barley? Black patent? Chocolate? All 3? What ratios? Who knows?
You absolutely will be able to brew good beer with extract. You absolutely will not know exactly what's in it. All you can do is make an educated guess, and keep tweaking until you zero in on your target. Which lasts until they reformulate it, or your LHBS gets a different shipment or changes suppliers or something like that, at which point you start over.