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We propose to use XMM to probe the z>~6 SMBHs mass growth rate during theirformation history. We calculate the equivalent Mseed (Msdeq) required to formthe SMBH assuming an exponential accretion at the Eddington rate. Hencehigh/low Msdeq implies a rapid/slow growth history. We find in our sample ofz>~6 QSOs a 2.5 sigma significant trend: the higher Msdeq the steeper thephoton index Gamma. This suggest Gamma as indicator of the SMBH mass growthrate. The proposed program aims at extending/improving the range in Msdeqprobed by our QSOs at both high and low ends. Our goal is to validate at ~4sigma level the Gamma-Msdeq relation and investigate the dependence of othernuclear parameters on Msdeq. The outcome of this program will provide new constraints for SMBH formation models. [truncated! Please see actual data for full text]