Personally, in an aeroplane of that class, I'd not bother.
I'd for preference use ground OAT and assume standard degradation with altitude, failing that or use the values from the latest Met Forecast.
For the 1-2 degrees error that *might* give you at altitude, the effects on the flight test data of an aeroplane in that class, the errors in any performance reduction will be trivial. (Realistically, the only really critical performance numbers for your aircraft will be take-off and landing, where a ground recorded OAT will be fine.)
Just avoid doing any performance climbs through, or above, an inversion - but there are plenty of reasons not to do that apart from inconsistencies in temperature measurement.
G