Just for clarity sake, you need to look at the mashout two ways: Direct heat (or herms) and infusion. If you have a way to heat to 170F without infusing to do it, it's all gravy. Why? Because sugars are more soluable in hotter water. If you can only do it with infusion, you're trading sparge volume for sugar soluability. In the end, it would be a breakeven at best and efficiency loss (ever slight) at worst.
It's further complicated by being dependent on your batch sparge method..
If you usually like one large batch infusion, the mashout WILL help efficiency.
If you usally screw up your sparge temps on the low side, a mashout WILL help.
Tex is close to right. I get a typical 90% efficiency with no mash out. I was able to bump that up to 92% when direct fire RIMS mashouts. When I've tried an infusion based mashout (which steals volume from my fresh sparge infusions), I've dropped down to 86%.
Disclaimer... everyone's system is unique and I encourage experimentation and obscene levels of data collection.