Is that the THP engine on the 308? When our 207 GTi was down on power there were no fault codes stored on it. It was put on a rolling road and measured 140bhp, that was after it had gained 10bhp from having the timing chain replaced.
We had the back of the inlet valves walnut blasted by a Citroen/Peugeot specialist in Hinckley and it was put on the rolling road again after this and measured at 174.7bhp, so almost the stock 175bhp.
Apparently BMW invented the machine that blasts the crushed walnut shells, PSA officially know nothing about it. To clean our car the only option they could offer was to take the head off
Jamie @ SP Tuning who carried out this work for us advised that he monitors his own cars and after having carried out the process on many cars, he reckons the walnut blasting should be done every 30k miles.
EDIT: It's actually quite easy to check the condition of the inlet valves. If you loose the inlet manifold you can stick a borescope down the inlet port and you'll be able to see what's going on. It's also easy to check the timing chain for wear, if you take the rocker cover off you can see the chain around the top sprocket. On ours you could wiggle it loosely with a screwdriver before it was changed.
P1385 is the variable camshaft timing solenoid.