You'll be having to battle with trial and fact to actually know if they'd benefit tbh, although you've more tyre to give you grip, you'll have more weight and rubbing (I'm sure you've already got a solution for this in mind though). Having just been moving; 245/35/19 and 265/35/19 alloys I can tell you they're extremely heavy and they were made by BBS for an M3.
To really get the benefit of wider tyres you need the right components in place to harvest the extra grip, you'll definitely feel an increase in cornering stability from them, but you depending on set up you could see Positive/negative or no difference at all to any track times. (If you were to track)
The main object would be maintaining grip under hard acceleration, in a straight line or exiting a corner - dialling off the lock. That's my main gripe.
Weight - yes, it'd be crucial to equal or reduce wheel and tyre weight to make this anything other than pointless. Some discussion about this earlier in the thread.
Rubbing - not an issue with the wider wings I mentioned in the original post. Without the availability of the Kerscher/SRS wings I might not have even considered this.
I agree with what you're saying about theory vs practice. While you could increase grip in general, there might be some undesirable side effects of a wider footprint (assuming all other variables like unsprung weight are kept in check). It might screw up the balance into the corner, off throttle, for example. I wouldn't want to spoil the recipe in that sense.
It'd be easy to get it wrong - increasing unsprung weight, neglecting geometry revision, etc. But there must be a way of getting it right. Or at least, moving a bit in the right direction.
