I will start this by saying that I am a firm believer that the proof of the beer is in the drinking and not in the rigor of measurements. However, when I change equipment I typically get inquisitive about its effects.
I started all grain using a 5 gallon Igloo cooler with a short copper pipe/gasket/valve in the bottom with a false bottom that I fashioned myself by taking a 10" SS bacon splatter screen and chopping the handle off and stuffing it into the tun. I would batch sparge by just pouring water from a pot into the tun. Worked great for two years (and the cost really appeased my cheapness!). The one time I did do the efficiency calcs I got in the 90's. Figured that was awesome and that I made a mistake somewhere. Beer tasted good and I went on about my business.
Just built a new mash tun using the Mainline Homebrewers instructions using a 10 gallon cooler and the stainless steal braided hose (minus the hose) as the false bottom. Works well from a seperation and flow standpoint but when I measured and calculated efficiency it was stupid low...like 50% low. I did the sample with the full volume collected and applied appropriate temperature corrections.
My general question is: Does the larger mash vessel affect batch sparge effficiency in some way? Since I have a tun and HLT now is it worth it to move to fly sparging?