" performance tyres " & a 23i suggest you aren't meaning a set that will cope with extreme speeds or trackdays :roll:
Ive been through many many brands in 8yrs Z4 ownership , you should really add wheel size & ideal budget but for the maximum performance minimum spend you can't go wrong with Vred Vorti or two others that really impressed me of late are the Dunlop SportMaxxRt summer tyre & Nokian WR4 ( won't be the cheapest of options) which i currently run as all seasons on my Z4Coupe & 5 touring F11 .