Could the increase in stall speed be linked to Reynold's number. Which states
R=(Rho*V*L) / viscosity
Where V=TAS, and L=chord length
Thus;
A reduction in density at higher altitude (all other factors the same as at low alt) will reduce R and hence reduce Cl max.
Therefore if you maintain the same weight (hypertheticaly) you would need a higher AoA at the same speed, thus less margin over the stall AoA, therfore you would stall at a higher speed????
I might be on the right track, but then again, it is 2:30 in the morning........
------------------
Eat My Vapour Trail