If I use 11.25 pounds of standard 2-row, 1.049 at 5 gallons is actually 61% efficiency.
Your mash ratio with 4.75 gallons is 19qts:11.25lbs = 1.68qts/lb which is a little thin but within reasonalbe limits. The grain will typically soak in about .1 gallons per pound leaving 3.65 gallons to extract. I can understand why the 2.5 gallon sparge was enough.
My MLT also has nearly one gallon of deadspace under the bottom. I shoot for a ratio of 1qt/lb then add one gallon. For your bill, that would be just under 4 gallons. You'd drain out about 3 gallons leaving two batches of 1.75 sparge.
To be honest, if you execute it carefully, you'd likely be able to hit the 1.050 target using 8 pounds of malt because your efficiency will go up because you'll be able to sparge more. Example: 8lbs with a ratio of 1qt/lb is 2 gallons + your 1 gallon deadspace. You'd get out about 2 gallons first runnings leaving two sparge infusions of about 2.1 gallons each.
Yup, the increased sparge volume really does make a big difference. This is why I advocate precise AG techniques because throwing a few extra pounds of malt into the bill isn't always the answer.