Here are some things to consider.
You have no control over how many other people drop their machines off for the same repair as yours between now and when you deliver yours for service. Staffing in the stores is geared to helping people and diagnosing - not fixing. Stores routinely send out involved repairs (and battery repairs on Macs are very involved now) - harder to do than a logic board or display swap or storage / RAM fix.
I always choose to go mail in repairs in the US now unless I have to take it to Apple. The repair centers are staffed for max parts, max labor and often they get repaired in less time than I would spend in the store parking / transit / queuing / handing off. Of course, you have overnight delivery so not all times is mail in faster, but for me it’s about 20 hours faster on average over the last 40 we’ve done for work. (We have about 3 repairs a week - most are damage and battery)
Yes - a tech could be ready, the parts could be there and you could get a same day battery repair, but with glued batteries, it’s not a 10 minute repair like hard drive swap, ram swap or battery swap used to be. It’s moving your computer from the old frame on to a new frame where the battery is glued on to the frame.
For any business that needs express service and loaners - check out http://jointventure.apple.com/