I too have the P- on order and am hoping its 3.2, but the reality is that some of that time is most likely attributed to the better grip from the 20 inch low profile tyres.
Time will tell, but I think on 18" aero's its likely to be closer to 3.5s
Don't over-think it! The road surface and weather conditions will have a far bigger influence on grip level.
If you are not happy with the ultimate grip level of the stock 18" tyres, just fit a set of performance tyres in the same size. For example the Michelin PS4 that comes with the 20" wheels is also available in the 18" size.
You might also be surprised to learn that ultra-low profile tyres are actually compromised in ultimate grip/performance and exist only for the "look". I would not be at all surprised to find that the 18s are superior on the same tyre (i.e. Michelin PS4) with the slightly more compliant profile. For a start you would be able to run a lower pressure in the 18s because of the larger air volume = more grip and more forgiving on the limit.
I have a professional motorsport engineering background (F1, BTCC) and have had this discussion many times with motorsport tyre designers (from Michelin, Bridgestone, Dunlop etc). Even done back-to-back track tests in BTCC with 18 v 19" wheels (although that was a long time ago, but the physics hasn't changed!).
From my own experience I was very happy to get the option of ordering a P-
I was actually dreading having to live with the 20" performance package for hooning around uk country backroads. The upgraded brakes would have been nice, but outside of track use I doubt they will be a big deal - unless the stock brakes are really that woeful, which I doubt.
To put grip levels in context, my barge of a Model X on standard 20" rims (45 profile tyres, same as 18" M3) with Michelin PS4 SUVs has more grip than you could ever safely use on a public road. Nothing has managed to drive away from it on our local backroads yet. So a P- Model 3 should be plenty awesome enough for any public use and even if a P+ did have more grip, you wouldn't be able to utilise it outside of a track environment anyway.