I make my PM recipes similarly - except I've started only boiling the mash and sparge runnings and adding all of the extract at the end (well 10 minutes from the end anyway), in an effort to get a little better hops utilization. I used to add some extract up front and few of the beers came out quite as hoppy as I wanted. They're good, I just want a little more from the hops so my last 2 recipes I saved all of the extract for last. Haven't tasted those yet so I can't really tell if it makes a noticeable difference.
Most of mine are around 5 lb of grains and a 3 gallon boil, so your larger boil should do a little better. Especially for an IPA I'd be inclined to move all the extract to the end but it should turn out fine either way.