I have 380 x 150 - 5 tires on my RV-8. On grass strips, gravel, even packed sandy surfaces like at my gliderport, the larger tires are great, a really noticeable improvement.
I would not say, however, that they rise to the level of back-country tires. I wouldn't land on sand dunes or gravel bars with them, or even a field full of squirrel holes.