slotting only for me.. helps when I have a shop and can basically unbolt everything and use a pry bar to get it as negative as possible.
People seem to be still gaining grip when they go slightly "over-wide" with the tires. For the limited and narrowly-defined use that you describe, losing some turn-in crispness and steering precision would be the downsides (that you compensate for from the driver seat, most likely by altering your timing of steering inputs to start a little earlier and relying more on slip angles to keep you running just wide of the cones instead of over top of them).
275's fit fine on the PP wheels (I ran those all last season). 285's also fit fine but I have those one a set of rims with the offset as far out as I can and it is close on the front strut.
The biggest thing my co-driver and I noticed was heat management. We aren't beating the crap out of the 285's nearly as bad as we were the 275's. The 275's would overheat on a single run with both of us driving and no amount of tire spraying would ever solve that problem because it was the carcass that was getting too hot and holding the temperature. With the 285's we get much better heat management and spraying actually means the tires will make it through a full run without issue!
This is something I noticed too this year I haven't seen any downsides from 285's but next year it is goodbye FS hello CAM
I'm not sure if you run the stock camber but if you slot the struts for more camber you will. Not much of a spacer probably like 5mm.
Maybe not. I'm running a 5mm hubcentric spacer but I have the slotted strut to spindle bolt hole camber mod. If you haven't done that and aren't running a crash bolts for camber (which I think is the new approved way to get legal front camber?) then you're probably going to be ok for tire to strut clearance.
I'd call it a good idea even if you don't actually track or autocross your car but do drive it with more than "moderate cornering enthusiasm" on a fairly regular basis. The only difference might be that you might not take the camber settings quite as far negative as you would for track/autocross.
I used a 5mm spacer as well but not because of tire clearance issues. I used them to help widen the front track to balance out some of the 1.7" wider rear track. What "help" this has is probably minimal but it makes me feel better! ;)