Also number of Poles in Berlin grows rapidly, so I don't think Warsaw will surpass Berlin
Officially Warsaw pop. is some 1.7 mln, but unofficially it's close to 2.1 mln now with metro pop. being over 3.2 mln.
What's more, if we add Lodz metro (some 1.1-1.2 mln) being (I mean metro area) less than 100km away, it's over 4.4 mln conurbation in Central Poland which can be compared only to Upper Silesia - Cracow conurbation on the south with pop. of over 4.5 mln.