I think you are saying you have 6.25 gallons of 1.078 rather than 5 gallons of 1.100. If you had boiled down to 5 gallons you would be around 1.0975--very close to your target.
You are just under the gravity for a barley wine--which is minimum of 1.080. If you insist on getting it to specs the only option really would be to add something. However, if you boil it, this means adding more water and really having a hard time. If it were me, I'd probably go with more sugars after giving letting fermentation get underway for about a day. Add it during heavy fermentation and there is little chance of any infection springing up. I would also aerate at that point again. Aerating a second time after about 18 to 24 hours or so is common with really heavy ales anyways. I guess the biggest question is what to add. I wouldn't add DME because that will just turn into a block of concrete in the fermenter and be almost impossible to mix in. LME adds more water, once again--though a smaller amount. Other options are candi sugar--which I really like and shouldn't throw you out of style. Adding things like this is not uncommon for really heavy beers.
Mind you, I have never done this. My personal practice is that if I miss going into the fermenter then I just have something that is not quite there and try to correct for the next batch.
What was your sparge/grist ration and mash temp?
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