When an aircraft is designed there is no way to find out if the maximum cruise speed is achievable with the chosen engine + wing + fuselage combination, and at what fuel flow.
Would it make sense to secure a section of high speed rail ( upto 320 kmh at least) to test full scale non flying models of light aircraft to get a better idea of what speeds could be achieved?
The fuselage could be sculpted and changed repeatedly in a short time to test various drag reducing measures and forces could be measured accurately in real time. Acceleration times to take off speed could also be measured.
Edit: It does not have to be a rocket sled, just a tracked vehicle with the full size aircraft mounted on it as it is in this case with the NASA full size wind tunnel:
For designers of kit planes and LSAs, full size tunnel time does not seem to be an option, I don't know. The design team do well to mount their plywood or GRP prototype with its engine attached and run in on the track, controlled remotely and with some capability for movement up and down and limited movement in roll and pitch. An on-board pilot may be an option.
To test a new propeller or a cowling, or fuselage shape, or retractable gear, wing design even, cycle times would be shorter, I think. The time to build a load bearing aerostructure, wait for good weather, time for take-off, climb and landing, not to mention test pilot time, all these would be reduced to zero - and postponed to a later stage once the design is finalized. All that is needed is to built a mock up capable of standing up to the forces on the track (2km should be OK) and the control system as well as the engine.
A typical test cycle would be as follows:
Build mock up Test run on track and obtain data Perform modification (replace cowl, wings, propeller etc) Repeat the above
Considerable time and cost would be saved on the aerostructure, pilot time, fuel cost and safety would be very much better. Handling and stall behavior also could be safely tested out.