This. If it was about dynamics youd have the lightest 17's you could find that fit over the brakes. Im a bit astonished that anyone has tried to say any different actually   
  
Im running 18's on the front and 19's on the back mainly because I cant find any front 19's that are wide enough   
The cost of the rear tyres also stops me drifting which could be a good or bad thing depending on your viewpoint  
  
I should add that the car is perfectly good on 19's, they dont detract from the handling or ride like they do on a 200SX, for instance  
I may be being thick here. You have 19" rims on back and 18" on front? 
I am still learning lol. 
That's why I started this post. 
My ultimate goal is to try and reduce the amount of back slide which has mainly occurred since the power upgrade. 
My method of thinking initially was a Quaiff Diff and the widest tyre that stick like Phoo to a shovel 
But these are just thoughts as Abbey can't set the car up untill all the other work is finished. 
Which makes sense. 
What I want to avoid it having to park it up during the Winter being to afraid to use it. 
Only done 3700 miles since I got it last July as it is. 
Not bothered about the cost of the rubber just the end name:)