I know the Jag system well, and I feel it's worth pointing out that "designed for" isn't the same thing as meaningly it'll happily take that all day long.
Back in the mists of time Jaguar fully intended there to be an X-Type R to compliment the R range of cars they sold. The X-Type R was to have been a supercharged 3.0litre of around 330bhp. 7 development cars were built. The torque bias was originally meant to be 70:30 in favour of the rear wheels on all 4x4 X-Types to make it feel as "RWD" as possible to make it feel Jaguarish despite being forced down the route of using an existing floorpan with a transverse engine.
What they found was that the failure rate of the Visteon designed transmission, and in particular the transfer differential assembly, was way, way in excess of what would be considered acceptable for a production car. That meant going back to the drawing board for the whole transmission, but Ford were in a bad way financially and decided to divert all R&D funding to their new F1 team. The X-Type R was killed, but even then the X-Type wasn't in the clear, because the n/a 3.0 V6 was suffering more transmission failures than is good for the reputation too.
The "solution" was to alter the sizing of sun, planet and ring gear in the transfer case to only send 61% of the torque to the rear instead of the original 70%. This got the failure rate numbers down to something Jaguar (and Ford) felt would be manageable through warranty claims.
I know nothing about the old Mondeo 4x4 system, but the point of this was to highlight that when running higher power than actually made production cars, you need to accept that you run a risk of a gearbox failure. It's not a certainty that it'll chew itself up, but neither is it certain that it'll be ok just because the original design calculations were based on a higher number. That's standard engineering practise, and it's done because it's hard to predict the effects of microscopic material defects and manufacturing flaws which are fairly common in low-criticality mass produced parts. Nor is it certain that just because one person has done it without a failure that you won't.
FWIW I've got the "painful" 3.0litre X-Type and I haven't had a transfer 'box failure. My old 2.5 X-Type didn't kill it's transfer 'box either. I ran a poll on the X-Type forum though and around 40% of the respondents had suffered a failure. Now I'm not stupid enough to believe 40% of X-Types break, as many only join a forum to get help with a problem. Even still, it's an issue which was never fully resolved, and if as little as 194bhp (2.5 litre) is enough break the odd gearbox, then you're going to want a spare transfer case in the garage if you intend on pumping more torque through it. It's not horsepower that matters - infact it's virtually irrelevant other than for input shaft RPM considerations. It's torque that does the damage.
