C 15 Fuel Filter Valve

I have read that when you get an ecm reflash that you should also close the fuel return valve at the filter base to force more fuel through the pump. If someone could please explain the mechanics of what this does it would help me to understand. I can see that it would likely have a positive effect on hp, but it seems that by forcing more fuel into the engine, it would have the opposite effect on mileage. I have run a couple days now since the tune, and with it closed, and while I for sure have more power, I don't yet know about mpg. One thing I did notice, is that the fuel in my tanks is no longer nice and warm like it used to be. It seems to me, that warm fuel in the tanks when you operate in a winter climate where temps can get easily to -40 is not a bad thing. I remember running through the dakotas and minnesota in the 80's during some nasty winter weather, and while many trucks were dead in the water with frozen fuel lines, my old screaming 8v92 detroit "green leaker" kept on chuggin, with warm
fuel tanks due to a lot of return. Feedback appreciated. Thanks.

 

Anyone else help on this one? The guy who did the reflash for me said to definitely close this valve. It doesn't seem to totally block the return fuel, just some of it. The fuel in the tanks is still a bit warm, just not as warm as it used to be.