after talking to Rana this afternoon about which gauge to buy i checked again if the pressure of 10^-5 Torr would be enough to not be limited by residual gas scattering.
It turned out that the simple number game i did last year was OK, but here a plot taking the full model (see P940008) into account. Matlab-file is in the svn (DYM: path/name?).

I only used hydrogen as a reference here. I'm too lazy to calculate the molecular polarizability for all the different gases for 1550nm (you need the refractive index for the gas at low temperatures to do so and then it depends on pressure and temperature).
Other gases will usually contribute more but i don't know how far i have to go as most of the usual ones taken into account (methane, CO2, Krypton, Xenon, ...) will be liquid already due to our low temperature so i think it doesn't make much sense.