Yes, this is a very good method.
I typically mash with the same water/grain ratio I'd use with all grain and put the mini mash bag in my oven for an hour at 150
After the mash I do a stovetop style sparge by dunking the bag a few times in 170 degree water and letting the bag drain
I'll combine all of that into the brewkettle and top of to my boil volume and then boil as usual, adding extract for the rest of my fermentables
I have done this many times and have had great results. Personally I don't see any need to do all grain since this method lets me mash any specialty grains I want and I can still do well on efficiency