We did something similar two weeks ago. On our last full day for the parks we had ADR's for 2:10pm at Crystal Palace in MK, but we still hadn't seen Light Motors Action and DH was not happy that we'd missed it. (there are only three shows per day).
So that morning we headed to the Studios, catching the bus from Pop Century around 9:30am. We arrived at The Backlot Tour at 11:00 and didn't have to wait but a few minutes for the show to begin. Just about 11:30 we were at the entrance to LMA, right as the CM's were roping it off saying the show was full. We kind of hung around, disappointed that we couldn't get in to the show and tried to figure out what to do instead, yet I wasn't convinced that the seats were filled to capacity. Whaddyaknow, there's room for more! When we made it around to be seated there was more than enough seating still available. I guess they had to wait for everyone else to be seated before they allowed more in. We were able to see the show afterall - and it was well worth it!
I suggest getting there at least 30 minutes before the show, or get a fast pass.