I have standard 7j 17"s all-round with 215/45/17 Tyres. Gives a slightly harsher ride than the 16" for obvious reasons. For the 300 they recommend 7.5j 17"s on the rear with 225/45/17 Tyres
http://teamlexus.co.uk/articles/UK%20Lexus...t%20IS300SC.pdf
If you are buying the car to modify, then 18"s and above look great, but to get the most you are then talking coil overs, sway bars, arbs etc...the list just gets longer. If you just want a slightly sportier look than the standards and slightly better handling then 17"s are fine.