Just ran a quick Rocksim of an Estes CC-Express with the booster lengthened to 107 mm (95 mm motor + 8 mm aft engine block). Loaded the sim with an imaginary E9-0 to E9-8. Mass is the same as it would be with an E12 to E9 combination, and the revised version ("EE-Express") is still stable.

Rocksim predicts 947 meters (3,107 feet). I imagine an E12 to E9 altitude would be slightly lower, but not much. Very impressive.

The CC-Express sustainer is also stable with an E motor in a 3 fin version. Rocksim of a 3 fin sustainer and 3 fin booster (same fins as used on the CC-Express) is also still stable with an E to E combination. Rocksim predicts 1004 meters (3,294 feet).

Very cool.