boost range 2.5tdi

okay was having power problems with my 2.5tdi Quattro,changed coolant temp sensor and this seems to have improved things. Its boosting from about 1500rpm in 3rd gear whistling away and pulling quietly and smoothly,but come 2500rpm the engine note changes and becomes louder the whistling stops and although the revs are still climbing i dont go much faster,it kind of flattens off .
4th gear is also the same. Is this 1000rpm boost window normal or should my car pull higher into the rev range as the redline is at 4500rpm. Oh and ive just dissconected the vac pipe on my EGR,but on or off it is the same. can anyone help, ?