That shouldn't be a big problem if you've got the individual samples into 24bit by 48000 sps. If Polyphone exports the soundfont to SFZ with 16bit samples, then just replace those with the ones of 24bit.
Polyphone is a great program for setting up the basics of SFZ-instruments. After that it's recommended to open the SFZ-file itself with a text editor to check if all desired parameters are correct, because in the arrangements to form a sampled instrument the SFZ-format is built up differently to SF2. (SF2-files are built up with samples → instruments → presets, while the SFZ-format only contain "instruments" and the samples are stored seperately.
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