What MINIAC said... Summer tires will start to lose traction at 7 Celsius and get progressively worse. You will also wear the tires faster if you are using them out of their temp range. I would highly recommend that you get some winter tires.
I had Bridgestone 17" RFTs on my car from new and they are OK in the recent cold weather and I have stuck to the factory recommended tyre pressures. I have assumed the sidewall won't deform enough to give extra footprint (if that is what you mean by changing the pressures for more grip). My MINI tyres have only done 6000 miles so are in good shape. I changed the tyres on our other car (Merc A class) despite being well within legal tolerances and found the handling and grip was transformed. May be worth looking more at the depth of tread than the pressures if you feel things are really out of hand.
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