In theory boiling for a good 60 mins is supposed to remove dimethyl sulfide which can add an off corn type flavour to the beer.
Also as you have already pointed out - you need less hops to achieve higher IBU's as your boiling for longer - however if you do boil longer then you'll loose more liquid to evaporation so you'll need to take that into account.
Personally, I made a beer which i only boiled for 20 mins and added heaps of Hops into - I called it "Lightening Ale" because it was fast to make!
I didn't taste any off or undesirable flavours in it at all.
In fact it was lovely