Well, first off congrats on your purchase..
Yeah those Nankang tires are not so good, from what I read they suk..
I'd say keep the tires on for now and whenever you have time replace them, as long as there's tread on them don't worry too much.. Now as for your next question, what size you should get.. First off if you have the IS stock 17's then you need to go with 17" tires because 16" won't fit.. The size that you already have on the car is fine, there's no need to change that unless you want to go a tad wider to a 225/45/17.. For tires I use Toyo Proxies T1-S's but those are for ultra high performance and I don't think you need or want that anyway.. But I know some people swear by Bridgestones.. Go to Tire Rack and check out the write-ups and make your own conclusions...