I don't think TN should have an impact on cylinder/engine life
I agree with all you say on this, but engine life is evidently not a black/white thing where if you do X then 100% of engines will make TBO without any work and if you do Y then 0% of them will.
It is a continuous progression, and I am certain that a NA engine lasts longer than a TN version of the same engine simply because the former spends less time running at a given power output.
Obviously one could operate the TN engine like one operates the NA one, but nobody is going to do that
Consider that the TSIO-540 can produce up to 350hp
True, though the MTBF on those is outrageous
(the TB21 is not worth it IMO, poor payload
Wait till you put TKS on one
The 500kg payload of a standard TB20GT drops to about 350kg with turbo and full TKS, which is roughly full fuel plus 100kg
A TB21 with TKS is a 2-seater, though a very capable one. But I wouldn't knock it because to expand the payload significantly beyond that, while retaining the range, etc, you need to buy a substantially bigger plane which will cost a packet more to fly.