As above, roll bars are a glorious mod - truly transformative IMO.
Tyres and wheels wise, I've done more than my fair share of experimentation

- settle in, it's a long post!
Dropping to 18s (or even 17s) can be very beneficial if you plan it out right as it can adjust the final drive ratio of the car - as alluded to by New Duke above.
DISCLAIMER - I AM TALKING ABOUT TRACK TYRES AND USE ON TRACK, NOT NECESSARILY APPLICABLE ON THE ROAD

- Tyre.png (13.84 KiB) Viewed 1153 times
Above is the comparison between a 225/40R19 and a 225/40R18. The former being the stock size for the R3. As you can see the circumference drops by nearly 4%. This, as a comparison, means that the wheel will take more rotations to cover the same distance; as a result of this it is essentially shortening the gear ratios, which in turn is inversely proportional to an increase in acceleration. It doesn't mean you'll get to 60mph 4% faster or anything like that, mainly because you'll be changing gears more frequently, but it does mean - in my personal experience - that the car will feel more sprightly and more 'zippy' on track. Drop down again to the 17s and you'll find a similar thing but the drop then goes to 7.7% in circumference reduction.
https://tiresize.com/comparison/ This is the website I use and it's really good for playing around to find what works. If you're running stock brakes then a set of 17s will fit, but it'll look a bit odd in the wheel well, especially compared to the 19s.
The other way to go about things is to change wheels for a wider wheel. I use an 9J width wheel on track and this means that I can run 265 width tyres. Jury is still out, having done only one track day with them, but the grip is insane. Only potential downside is that the level of grip outweighs the car's power a bit, so it loses some of the pivot control on throttle compared to a skinnier tyre - but my laptimes at Cadwell were improved on my previous, so they're faster, just not as engaging a drive. As said, only from a single track day, in the dry, though so can't comment much further.
On the choice of tyres, my personal preference would be to get a spare set and go for something you don't mind burning through. Cup 2s are mega, but they also cost mega money... I run Accelera 651 Sport tyres and find them great, I've run AD08R and NS2R as well and find the 651 to be the better of the 3. That being said, the AD08R has been replaced with the AD08RS which I've heard bad things about.