Im a noob at this...
 

Im swapping my stock wheels and tyres for 18x9.5 XXR527 matte black wheels with Kumho KU31 245/40/18 tyres. Would swapping it make my car slower or faster? Im a noob at all this traction and rotational inertia stuff haha...

im not a pro either but your question could have a slew of different answers. are you tracking your car? yes? then your track times might suffer slightly. are you just daily driving your car? then it doesnt really matter does it?? the xxr's will look better than stock so enjoy that

Slower both in a straight line and round bends.

You are changing the gearing by fitting larger diameter rear tires. Your speedo will look like you are going even slower than you really are. You will be 8% slower.

Wheels wider than 8 inches on a tire that is of correct size will substantially hurt cornering behaviour.

Largest wheel and tire that will improve things is likely 18x8 and 225/40 x18. TRD Sport version just released in the UK fits this package although Evo magazine reports a 225/45x18 tire size which, if true would require a speedo drive change and would definitely slow the car down.

Your proposed package will be cosmetic only.

Anyone who actually knows the effects of what you have in mind will know you are not a serious driver.

Your tire choice is taller than stock diameter which will slow your acceleration, it should be 245/35/ 18, not to mention wider tires and wheels most of the are heavier than stock setup, the right tires size is 245/35/18 and 255/35/18 if you want less stretched tires, cause 245 are for 9.0 wide wheels.

You can compare sizes here http://www.frsproject.com/tire_size.php