52,369m on first attempt. 52,396m on second. Edit: I got it up to 52,656m, but only if you allow a single decoupler on the bottom to remove the stickiness.
So that 248km was using your design. I tried out my own design and managed to get 254 km. I think I might even be able to tweak it a bit to get a little more out of it. Edit: I've managed to take it up from 254.2km to 254.4km. Edit2: Well, I just crushed those: 262.5km