Sorry 'Smells' I misunderstood when you said you had a "non stock exhaust" I immediately thought
headers as well when you mention cast headers on an A15E "but I dont have one of those ".
It's bit hard to picture the actual issue without a photo - is that possible ?
I wouldn't be too woried about putting it after the join, if thats the only position available, as some tuners use a 'sniffer' O2 sensor which is just an O2 sensor shoved up the tailpipe. Just has to be mounted above the horizontal ( I think its to avoid condensation contamination )
Permatex ULTRA copper is sensor safe
https://www.mxstore.com.au/p/Permatex- ... PonKoZ68nopsaAs1pEALw_wcB