I just got one in the 1400s as well, didn't think to screen cap it.
A few problems appear at speed: The first is altitude control. Keeping it under any specific altitude is more luck than anything. The second is that your aerodynamic nosecone, which you need to go that fast, blows up, dropping your speed.
Roll issues also appear, but they can be mostly mitigated by taking an elevon and turning it on its side.
Craft is basically: nosecone, your choice of probe bodies, engine pre-cooler (both intake air and fuel, and it's light!), a FL-T100 fuel tank, a rapier, some wings and elevons to keep the thing in the air.
You'll also need a few seperatrons and a launch stabilizer to get it in the air.
edit: radiators do fix the overheating problem, but I think the airbreathing rapier runs out of power at ~1400m/s