Power Loss due to exhaust

Hi, my 2.0t exhaust is blowing near the front of the car. I've been told the part where it clamps together at the front has deteriorated and is being replaced tomorrow.
There has been a noticeable drop in power delivery and turbo boost. Is this related to the lack of pressure due to the faulty exhaust?
Cheers
Chris

It was a clamp upfront which was replaced. Only cost me £30 and he shoved some silicon on the y pipe where the exhaust splits into 2 at the back.
The car definitely feels quicker since it has been done, but not 100% yet. I can feel it pull ok in low revs in 2nd, but after that it doesn't feel as quick as it did before the engine rebuild. It is definitely a lack of pull from the turbo. It feels a bit like before when I had the diverter valve changed, but better.
I'm wondering if the fact that the knock sensor needs replacing could be causing this.
Also, does anyone think that there would be any reason to tamper with the turbo's diverter valve during an engine rebuild with new cylinder head? (Turbo was fine)