Why would there be an issue of Ba’al Tosif? Technically, tzitzis (other than the required p’sil techeles) can be any color – there is no requirement for them to be white. As such, even if the current version of techeles is not correct, Ba’al Tosef would not be an issue. I could see that yuhara could be an issue, whether it is correct or not (though more of an issue if it’s not), but even that is questionable.
As an aside – Rav Yisrael Belsky, Z”L, personally told me that he thought the current techeles from the Machon P’til Techelet is correct. He further told me that the only reason he did not wear it publicly was on account of yuhara – so he only wore it on Shabbos on his Talis Katan, when it would be hidden under his Kapoto.
an Israeli Yid