My buddies and I did 3 PM batches last year to transition to AG. The three recipes had wildly varying grain bills in the mash; one only had 3lbs of grain, while another had 6lbs! They all came out really well
I'd say you could probably up the quantity of grain if you want, but it may or may not be worth it- we didn't check our efficiency on any of those three batches, so we might not have NEEDED the larger grain bill (it's possible we were not getting everything we could have from the grain). It sounds like you crunched your numbers- we were pretty casual about volumes- so take this all with a grain of salt.
It definitely seems to be because of a small boil size (we were in the same boat). For the larger bill, we were afraid we'd wind up throwing out wort and skimped on our sparge.
But in the end, all three of those batches were far and away the best beers we've made so far. We just did our first AG Monday and we're dying to see how it comes out! Good luck!