Does anyone know why the turbo boost is held back until the engine reaches 2500 rpm on the 2.5 turbo?
I would like to increase the boost at lower engine speed if possible.

you just have to understand what is going on.

The turbo "spools" up from the hot expanding exhaust gases that are coming out of the engine. As the turbo spools up the rpm goes higher and higher and the intake side of the turbo turns into a compressor. At low speeds it is not turning fast enough to compress the air in. Read all about it here.

http://auto.howstuffworks.com/turbo.htm

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Jim

If your turbo has an air recirculation valve, that could contribute to building boost more gradually.

Sixto
91 300SE
87 300SDL
83 300SD

According to my 602 engine manual you should be reaching full boost by 1600-1700 RPM. You may have a problem with your boost control hardware.

Tim