The compressed air leak sound could possibly be a compression leak past the prechamber, i have had to different 2.5 turbos have a prchamber leak. Look around the injectors for the leak and if the plastic dirt protectors are still there they will start to melt a litte on the leaking ones, you might see a wet area around the injector, if it's realy bad u can feel it with your hand.

As for being slow, i bet your turbo is not producing boost, do a search on here for GXSR's pressure operated wastegate conversion, it's worth doing, i did it!! These cars are plaged with leaking vacume systems and the turbo wastegate works off of vacume. If u do this i would also delete the egr.

Also with 300000 miles, i would def check the timeing chain to see if it's streched (if it's really streched this throws off the timeing and can affect how it runs). Also have the injectors checked out.

Hope this helps

__________________
2011 e320 cdi (132,000) current
2005 Jeep CRD diesel--2.8L (138,000 miles) wrecked, insurance took
1993 Mercedes benz 300D 2.5 Turbo Frybrid powered. (336,000 miles) wrecked and sold as parts car
1990 Mercedes benz 300D 2.5 Turbo (170,000 miles) wrecked and sold as parts car

Check out my Frybrid Vegtable Oil Conversion at http://1990300dfrybridconversion.shutterfly.com/