okay-I won't have time to continue with this problem for a few days, so I post what I tried if it helps to another person:
I tried with WF6, is a corrosive gas, density about 12 (that doesn't match) but it reacts with water to give:
WF6 + 3H20 = WO3 (yellow, dissolves in NaOH) + 6HF
Then HF produced reacts with SiO2:
4HF + SiO2 = SiF4 + 2H20
according to stoichiometry I got that, for each mol of WF6 that reacts, we get 3/2 moles SiF4 gas, that would give me the 50% increase in pressure.