I've just noticed a boost leak when doing a BLT, it was connected on the TB Elbow and pressurized but i couldnt increase pressure beyond 4/5psi. there was an airleak which i guess is around TB.

i want to know if there is a leak in TB, will it prevent boost from going beyond certain psi (say 1bar)??

Thanks

You just answered your own question

The turbo will work harder as well.

Use a spray bottle of soapy water and spray well, soak anything that has any sort of pressurized air or vacuum through it. To your question, it's either the gaskets, the shaft seals or the BISS.

Yes, a boost leak is not allowing the pressurized air to get to the engine so you won't be boosting how you should. Example for every PSI of boost, the turbo spins at 100RPM. So for 15PSI you would spin at 1500RPM at the turbo. A boost leak would cause those numbers to be off and could now look something like 5PSI at 2500RPM. All because the boost is leaking out and your motor is only getting 5PSI out of the 25PSI the turbo is making. That will all depend on the severity of the leak/s you have though.

Spray it with the soapy water as said to find the exact leak. That must be a pretty nasty leak if it leaks out past 5psi.
Fix it asap and enjoy the increased performance
Writeup for the shaft seals can be found on vfaq.com