Hey Grant,
I'd most likely suggest running the 17's if you are looking to run something aroung the 25.7 Overall diameter. There really aren't too many great options in the 255/40R18 other than the Advan Neova. You might want to consider running something in a 25.0 OD. That way you can run the 255/40R17 or even the 255/35R18, which will give you a lot more tire options.
The Dunlop Star Spec Z1 is an absolute great buy for the price. I wouldn't go as bold as saying its a better tire than the Neova. Most of the track reviews and times show that the Neova outperform the Z1. Really tho, the only downside to the Z1 will be the wet traction performance.
The Federal 595RS-R is also a great buy. The problem is... they've been such hot tires for us to sell that its been virtually out of stock most of the time.
If you want to save some valuable cash, a good idea for beginner/novice drivers or cars that people are just trying to dial in... is to get a good tire, not a great one. Save some money, and get the car dialed in. It doesn't make much sense to spend $300 a tire to get 0.01 seconds off your time. If you are just trying to setup your car, I think you are on the right track to try not to spend too much on the tires.
Hope this helps
