Is there a "high altitude" aerofoil ?

[IO540]

Have you worked out the lift coefficient of your TB20 in high altitude cruise? You only need the mass, wing area and IAS.

1400kg
11.9 m2
95kt

from here.

The Mirage data from here is

1950kg
16.3m2
and I guess Vbg is going to be around 95kt too.