Simple advice - if you are a newish ppl holder, dont fly the aeroplane with ice on it. Some aircraft are more tolerant than others, but why take the risk? I disagree with IO540, trying to put a figure on what is an acceptable amount of ice on a PA28 can lead to deaths, thats why its not in the POH - how do you measure the ice? where do you measure it? Those are all valid questions. Why not just follow the POH? CLEAR the ice / frost off before flight. If you are in flight and you get some ice that was not predicted, do something to change whats happening before your trying to fly a giant ice cube, climb, descend or turn back, dont blindly push on in an aircraft not equipped for ice thinking it will get better. Its too big a gamble.
If you want extreme for airflow disruption due to objects on a wing, try the Rutan Longeze, the SLIGHTEST precipitation in the air reduces cruise by 10kts, and you often feel the aircraft trying to pitch nose down with a reduction in airspeed BEFORE you even see the visible precipitation.
To say that the TB20 has a similar wing to the PA28 is even more ludicrous of IO540. Even small changes in planfom, aspect ratio, chord thicknes or most important of all airfoil section can have dramatic effect on an aircraft performance and tolerance to different things.