I don't believe the gearbox numbers 13,15,17, etc actually correspond to any given amount of torque it can handle. For example, F13 and F15 gearboxes are the same internally, they only differ in the fact the F15 uses larger and stronger CVs than the F13. F13 and F17 boxes use the same inner CVs I believe.
As for how much the Getrag boxes handle, they can handle a fair bit of power all depending on your clutch setup and more importantly, your launching technique. Having upwards of 300hp going through a F17 and repeatedly dumping the clutch isn't going to your gearbox any favours. I do remember @gman saying something like this in regards to a F23 gearbox that is usually found in the Astra SRi turbo and some other models with 2.0/2.2 engines (turbo and non-turbo boxes here differ slightly too I'm led to believe).
If you need a gearbox upgrade with the addition of forced induction, I would look into whether a F23 (would have to be from a Z20LET, C20SEL or C22SEL, these would have the Family II bellhousing pattern) could be fitted to the Z18XE, as I know some of the earlier F16/F18/F20 boxes (which were for the Family II engines) could be bolted to the Family 1 blocks. And the F23 box fits in a Corsa C with the gearbox mounts from the European Corsa C SRi 1.7CDTi diesel (which also uses the F23 box, albeit different bellhousing pattern). You'd need the matching shafts/CVs to fit, the shifter mechanism and cables from the SRi diesel. But then, if I were going to the hassle of turboing a Z18XE and needing to fit a F23, I'd just fit the matching Z20LET with it and ditch the Z18XE altogether.