I have heard conflicting reports on this, re the TB20.
Some owners say it does reduce speed. The STC however says, AFAIK, there is no performance change.
The leading edge component is about 8mm thick which would seem to be insignificant, but one can never tell.
Well, the TKS system is a sort of fine mesh on the leading edge, so one could imagine the airflow there to be not as smooth as with a solid leading edge, hence somewhat increased drag = reduced performance. A friend of mine owns a TB20 with TKS, if you want PM me and I'll try to get some performance figures from him.
The TKS I have seen is a solid sheet of (what looks like) titanium or stainless steel, about 1mm thick, with tiny holes drilled in it. I can just about see the holes with my eyes. So it is smooth enough. I would guess any loss of speed would come from the fact that the leading edge is effectively getting a "bump" stuck on the front of it.
There may be various models / version, the one(s) I have seen were definitely visible and, as said, resembled a very fine mesh.
Different but relevant to the "bump" argument, on aircraft with boots, normally activation leads to a speed reduction due to drag. Of course they deform the edge more.
The correct answer is "Yes it does".
You may expect up to 3-5 knots. The POH also claims loss of climb performance.
Do not ask me why
The TKS system reduced high cruise speeds by about 7 KTAS on a Mooney Ovation 2GX I flew two years ago. I base the comparison on several reports stating that non-de-iced Ovations performs by the book. Being a low-drag airframe, the Ovation probably suffers more from the installation than other, less aerodynamical airframes.
On the DA42 there is no difference. People from Diamond told me that, and hence there is nothing in the AFM. I also have compared speeds between non-TKS and TKS planes here on the field, and there is no change, but of course that is just one data point. All this is probably caused by the fact that both versions have a similar leading edge. With TKS there is a silicon-filled gap between the TKS and wing sections, and on the non-TKS there is that same silicon gap. The 'TKS-portion' of the non-TKS version is composite, but is an insert to the wing. Hope I make myself clear..
Maybe on other aircraft the wing is in 'one piece' which could explain speed differences.
One of the speed increase measures that Diamond implemented on the DA42 VI was to smoothen this silicon gap.
I used to fly X2 PA44, one with TKSand the other without. The one with the TKS used to be ~ 5kts slower.
5kts is a LOT of power lost.
I am really suprised.