I agree with Firas, that if Silva is pursuing the GSP fight very actively, he should be the one who comes down in weight. I know 170lb would be very difficult, but the halfway mark of 177.5lb is very reasonable.
But given how Silva said he would never fight Jones unless JJ met him at catchweight (knowing very well it would then be disadvantageous to JJ), and forcing GSP to move up to 185lb (giving Silva the advantage), it's very clear that Silva wants to attain every advantage possible.
Firas is not only one of the best coaches out there, he is also GSP's best friend, and has stated he would never send GSP into a fight that he wasnt ready for or one that would be disadvantageous to him. Seeing as how GSP has never fought at MW before, and fighting Silva, who would have every size advantage on him, I think Firas would advise GSP not to take the Silva fight, unless it was a catchweight bout.