We think alike Modnrod. I plan the same kind of no-clamp slip joint. Turbo charged light aircraft use this set up because its reliable and light. Since the inside pipe runs hotter it expands to make a good seal.
A diametrical clearance of .005" to .008" should seal well when hot but also allow some movement. I measured the pieces from the cut up KTM header and four Ducati mufflers and found pieces that have this clearance.
Current Stable : 950SM Turbo! GSXR1000 RT-P1100 VFR800 RXC620 motard EXC520 CR500AF CRF450R Plated! CR250
Past stable : 90 +