Just an idea that I used once apparently to good effect. I sparged a similar sized mash bill but instead of going into the kettle with the rest of the wort, I kept it as a separate batch, and added some LME to bring up the gravity. So the main batch was full strength mash, the sparge became a new batch with gravity boosted by extract. The sparge / extract batch I now see came out pretty good and I forgot what I did with it or what it tasted like because at the time I put it up, I actually didn't taste the fully conditioned beer. The main batch went to keg as usual, and I bottled the sparge batch and forgot about it. 6 months later I am moving my beer stuff to the new house and find a case and a half of bottled mystery beer and I wasn't supposed to have any in bottles. I chilled and tasted a couple and it was pretty good. Not my best, but good enough for dinner table beer. I couldn't remember where it came from, though. JUST NOW I put two and two together and remembered what I did, and this must be the sparge batch. I will try that again some time down the line. The next two brew sessions are already planned in detail.
There is no point in throwing away good fermentables IMHO. If you are considering a no-sparge batch, I suggest you go ahead and sparge two gallons but keep it out of your main batch, chill a sample and check the gravity. Add enough suitable extract for an acceptable OG, give it a boil, ferment separately, and see what you get! You should have a lot of the complexity of your mash bill superimposed on a base of extract that is essentially your blank canvas.
