Welly wrote:20bhp but more importantly
more torque - this is what kills clutches.
You are right though you would think that a small increase in power would be ok but these days everything is made down to a price so if it doesn't
need to withstand 130bhp then it won't be
made to either
Taken to an extreme; if you managed to get 260bhp from the HDi then it would blow the headgasket, the turbo, the gearbox all very quickly

To put the situation on its head, would you pay an extra 50-100% for a car with 110hp, but with every part specified to be able to handle more than twice that? Wouldn't you be wondering "why am I paying for something I'll never use?" or "why don't they just sell it with 260hp, then?"?
The 406 can handle 210hp perfectly well, otherwise owning a 3.0 V6 would be a nightmare. But over-specifying parts to such a degree as to handle a 260hp diesel (with 5-600nm torque) is expensive, and not a very sound business idea when 99,999% of your buyers will never even remotely approach the operating limits of the car at 110hp, let alone at 260hp.
Plenty of 406s reach 300.000km and even 400.000km, I've seen examples for sale with over 500.000 on both petrol and diesel engines. They hardly ever rust, parts are generally inexpensive and they're simple enough that any mechanic can fix them. Personally, I'd consider that as overbuilt as PSA could possibly justify without bankrupting themselves. Then, if you want to go beyond the performance envelope set down by PSA, you need to buy uprated aftermarket parts. I would say that's rather reasonable :-)