I wouldn't even boil a tin of extract. It's already been boiled twice, once to make the "wort", the second time to evaporate the excess water. Although that second boil is done under vacuum I reckon it's been boiled enough. I'd just boil a small portion in water in a pan with hops and strain that into the fermenter with the tin warmed until runny, stir that up with water in a stockpot or large pan and chuck that in fermenter as well.
Ive done a couple batches with pre hopped tins and added DME, I try to add the Can as late as possible but the end result always seem to be more bitter then i'd hoped, im guessing raising of the heat is reboiling the aroma hop additions and resulting in a more bitter beer.
If you boil it long enough you drive off the hop flavour and aroma but retain the bitterness (Charst's evidence above suggests it may even increase bitterness). Some might see this as worth doing to create a fresh palette on which to build your own beer - I'd suggest a brewer should just use unhopped extract instead.