Agreed, buying in bulk will save you a bunch in the long run. Here, bulk costs me $1.00 - $1.25 for base grains as opposed to $1.50 - $2.00 buying per pound. So ~ $5/batch on average. My other big savings is from washing yeast.
Back on topic, yes, 3-8% extra from dunking is what I've noticed. This isn't huge, once you translate into actual gravity points, but is something. Most may not care for the extra effort, but it happens to work out well for me due to my kettle sizes. I'll emphasize again though, crush makes all the difference in the world, particularly with BIAB. The best advice I can give to a BIABer is to purchase your own mill. This, and to a lesser extent, dunking, got me from low-mid 70's to low-mid 90's (still get upper 80's for large beers). You'll probably pay for your mill in grain savings over the course of a year or two, depending on how much you brew. And, you'll probably want it when you start buying in bulk anyway.