My experience with the 20's is that they are capable of absolutely jaw dropping planetary views under the right conditions.
I'll relate one observing session that really changed the way I look at designing and building scopes. On this particular night, the wind was really strong... I mean so strong I elected not to use the shroud(s) so the scope was basically open frame. Where I was set up was somewhat protected by trees which cut down the general force of the wind, but still resulted in cyclical intermittent gusts from the vortexes generated by the wind brake. Essentially, the local conditions went from no-wind to medium wind to strong wind and back again over a period of a few seconds.
With no wind, the seeing was poor... normally I would have put that down to just a crap night, and thought no more of it.
With medium wind, no change... still crap in both eyepieces. Now here is an important lesson, if the seeing conditions were ONLY the result of body heat, the wind would have preferentially pushed the plume to affect just one of the OTA's. There was no discernable difference between either OTA.
With strong wind...
The image sharpness just locked down to perfect. I can say without a shadow of a doubt that I have never before (or since) seen astronomical views remotely as good as that through any scope ... period.
The lesson here is that 90% of what I had thought of as bad seeing could be traced to the boundary layer.... that pesky plume of heated air 1 inch in front of the primary. Therefore, thermal management should be prioritized pretty much above all else.
For some interesting reading on how the pro's do it, Wilson is a pretty good place to start. It has been a while since I read it, but from memory he advocates an airstream flowing at 2.5m/s (or was it 1.5? I forget) and curiously, suggests blowing horizontally across the optic is better than having your fan aligned to blow vertically up or down.
I guess the other point to note here is that whatever issues are associated with a loss of accuracy in an optic's figure due to temperature gradients, any loss of image sharpness pales in comparison to the convective plume in the optical path.
2c
~c