Agree there! It is hard to take a chip designed to just squeek by some emissions requirements at one band and upconvert it to another band. At the least, you are going to have to do some additional filtering, step size reformating, and modulation reprogramming. For example, what is a good modulation bandwidth at 433 might be too small a bandwidth at 1600.
I would start by getting the 433 chip on a protoboard and hook it to a spectrum analyzer and pretend it is working at 1600, and see if it will meet the specs. Will the 2nd/3rd harmonic upconvert to a band you do not want to interfer with?
Also, if it is a low power chip, upconverting and amplifying to a higher power might bring on much more stringent specs to meet--ones the original chip is 20 dB out of spec for!