This car will curb boost when it hits it's torque targets. Basically once you reach a certain power level it curbs the boost. Maybe you're lucky enough to hit that limit at the lower boost. Wouldn't hurt to check your clamps though.
Here are the connections you’ll need to check, from FiestaST.org: Connection 1 – On the back of the engine looking up from underneath, there is a connection of a rubber hose going onto the outlet of the turbo charger. Connection 2 – The other end of that rubber pipe goes down onto a thin looking metal pipe. Connection 3 – This is the other end of the metal pipe. From under the car these 3 are easy to access. It connects the metal pipe to the rubber hose that goes to the intercooler. Connection 4 – This connection is on the passenger side of the intercooler (looking head on at the car) a lot harder to access without removing the fascia. Connection 5 – This one is the driver’s side of the intercooler. It can be hard to access without removing the fascia. Connection 6 – This connects a rubber hose coming from the driver’s side of the intercooler and goes onto a plastic pipe that heads up towards the intake. Connection 7 – This is the other end of the plastic pipe, It goes to a short peice of rubber hose just before the throttle plate (Hard to get to will probably need a 7mm wrench, not a socket) Connection 8 – This is directly onto the throttle body. This can be hard to get to with all the AC pipes in the way, but not too hard.
I was able to get to all of them except the one on the drivers side inner cooler... But does anyone know how tight these should be? I thought they were pretty loose based on my 'feel' but i have no idea how tight they can go without stripping out the clamp or breaking something.
Just for the heck of it, I decided to check mine. There are 10 in total. Of the 10, 3 were so loose I could turn them by hand. The others needed at least 5-7 turns with the ratchet to fasten. You will need ratchet with a 7mm socket.
Did a test run after the fix, and according to TorquePro I am now getting 24.4 Psi on overboost, my previous max was 16.2 Psi.