If you're focus is track days, then another big factor is the availability of tires. There are no good tires available in 170.
In 160 Dunlop has nothing except a slick, Pirelli has lots of options and I'm not sure on Michelin.
180 has the best choices of all, track day, racing DOT and slicks. It is also where all the "development" work from racing trickles down to the fastest (along with 190s).
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