Interesting discussion.
I use ecopia EP150 165/70 R14's as my summer tyres, ContiWinterContact during the winter. I felt a huge difference when we changed to our wintertyres, FE did fall a little, but drivng in snow, or even on a cold wet tarmac, the ContiWinterContact feels much better planted and therefore a lot safer. I would definately recommend having dedicated summertyres and wintertyres over all seasons! People think all season tyres are better in the rain than summer tyres, but that is only true for tyre temperatures under 7 celcius (44F). All season tyres will always be a compromise!
I've been looking at changing my wheelsize, and that, of course, also means changing my tyres. I have some 14" alloys, but as i've later found out, they look light, but are actually not that much lighter than steelies. Therefore i've been looking at some ROTA Slip Streams 15", they are quite cheap compared to anything else on the market with such a low weight, and they look alot more aerodynamic than my current wheels. Going from 14 to 15 would also mean that i can get away with only a small increase in tyre width (165->175), and increase my rolling radius by 3,7%..
I found a great online tool for calculating and comparing different tyre sizes here
Visual Tyre Size Calculator | Kouki Tech