If we're talking about a factory effort where margins are small, you can't just add displacement to an existing engine. It's needs a clean sheet design and then redesigend chassis too... it's too late for such rule changes. Too late also because regulations come in 2014.
Plus, they'd still have to tweak the restrictor sizes (and boost pressures) for all other configurations of petrol engines because in theory they should all make roughly the same power.
Btw, there are no restrictions on the amount of cylinders but with the discplacement limited at 2.0 litres for turbocharged petrol engines, V10 is not very viable option.
Ultra Lightweight Technology is just an ad on the car.
I bet they both weight roughly the same which is under the minimum weight 900 kg (under because they probably want to play around with ballast placement).
Audi uses single turbocharger and V6, Peugeot twin-turbo V8. That's going to be a bigger difference regarding the restrictor and boost pressure cuts.