@jancellor @vicb Thank you for your input. I'm sorry that I never discovered your thread before.
The Sounding forecast already uses the surface temperature/dewpoint at the 1000hPa level. (or perhaps it uses the temperature and dewpoint from getPointForecastData which gives the same values). This is plain to see if you display the temperature, dewpoint and LCL separately in another add-in
My understanding is that "surface" means the air in contact with the surface (normally at 2m). Indeed dewpoint-surface, rh-surface, wind-v-surface etc are clearly properties of the air close to the surface rather than the surface itself.
I find it impossible to see any good reason not to use the surface temperature/dewpoint at the surface level (model elevation).
Personally, I see this as quite an important bug in the official sounding forecast.