It's a well known fact that you can't hear much above 15Khz but you can certainly hear a super tweeter in operation - it adds air and space and is difficult to do without once you have heard it.
Back in the early 80's I used to have a pair of cheapo Piezo super-tweets on flying leads with croc clips on the end. I used to tack them on to any and every speaker I got my hands on and most agreed that it was an improvement. I don't think it cost more than 8 quid to do....:)

